cmd/go: move module cache from $GOPATH/src/mod to $GOPATH/pkg/mod

Using $GOPATH/src/mod confuses too many tools.
$GOPATH/pkg/mod seems better for now.
It's also next to dep's cache, $GOPATH/pkg/dep.
If we do eliminate GOPATH/pkg for holding .a files (#4719)
then we could still keep it around for pkg/mod.
(Or we could move the module cache again then.)

Fixes #26401.
Fixes #26635.

Change-Id: I18f7da216ed9f490eded3c00d837fb086ae5b6a4
Reviewed-on: https://go-review.googlesource.com/126755
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/internal/clean/clean.go

@@ -153,10 +153,10 @@ func runClean(cmd *base.Command, args []string) {
 	}
 
 	if cleanModcache {
-		if modfetch.SrcMod == "" {
+		if modfetch.PkgMod == "" {
 			base.Fatalf("go clean -modcache: no module cache")
 		}
-		if err := removeAll(modfetch.SrcMod); err != nil {
+		if err := removeAll(modfetch.PkgMod); err != nil {
 			base.Errorf("go clean -modcache: %v", err)
 		}
 	}

src/cmd/go/internal/help/helpdoc.go

@@ -377,7 +377,7 @@ See https://golang.org/doc/code.html for an example.
 GOPATH and Modules
 
 When using modules, GOPATH is no longer used for resolving imports.
-However, it is still used to store downloaded source code (in GOPATH/src/mod)
+However, it is still used to store downloaded source code (in GOPATH/pkg/mod)
 and compiled commands (in GOPATH/bin).
 
 Internal Directories

src/cmd/go/internal/load/pkg.go

@@ -434,7 +434,7 @@ func LoadImport(path, srcDir string, parent *Package, stk *ImportStack, importPo
 
 	if strings.HasPrefix(path, "mod/") {
 		// Paths beginning with "mod/" might accidentally
-		// look in the module cache directory tree in $GOPATH/src/mod/.
+		// look in the module cache directory tree in $GOPATH/pkg/mod/.
 		// This prefix is owned by the Go core for possible use in the
 		// standard library (since it does not begin with a domain name),
 		// so it's OK to disallow entirely.

src/cmd/go/internal/modconv/convert_test.go

@@ -39,7 +39,7 @@ func testMain(m *testing.M) int {
 		log.Fatal(err)
 	}
 	defer os.RemoveAll(dir)
-	modfetch.SrcMod = filepath.Join(dir, "src/mod")
+	modfetch.PkgMod = filepath.Join(dir, "pkg/mod")
 	codehost.WorkRoot = filepath.Join(dir, "codework")
 
 	return m.Run()

src/cmd/go/internal/modfetch/cache.go

@@ -22,17 +22,17 @@ import (
 
 var QuietLookup bool // do not print about lookups
 
-var SrcMod string // $GOPATH/src/mod; set by package modload
+var PkgMod string // $GOPATH/pkg/mod; set by package modload
 
 func cacheDir(path string) (string, error) {
-	if SrcMod == "" {
-		return "", fmt.Errorf("internal error: modfetch.SrcMod not set")
+	if PkgMod == "" {
+		return "", fmt.Errorf("internal error: modfetch.PkgMod not set")
 	}
 	enc, err := module.EncodePath(path)
 	if err != nil {
 		return "", err
 	}
-	return filepath.Join(SrcMod, "cache/download", enc, "/@v"), nil
+	return filepath.Join(PkgMod, "cache/download", enc, "/@v"), nil
 }
 
 func CachePath(m module.Version, suffix string) (string, error) {
@@ -54,8 +54,8 @@ func CachePath(m module.Version, suffix string) (string, error) {
 }
 
 func DownloadDir(m module.Version) (string, error) {
-	if SrcMod == "" {
-		return "", fmt.Errorf("internal error: modfetch.SrcMod not set")
+	if PkgMod == "" {
+		return "", fmt.Errorf("internal error: modfetch.PkgMod not set")
 	}
 	enc, err := module.EncodePath(m.Path)
 	if err != nil {
@@ -71,7 +71,7 @@ func DownloadDir(m module.Version) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	return filepath.Join(SrcMod, enc+"@"+encVer), nil
+	return filepath.Join(PkgMod, enc+"@"+encVer), nil
 }
 
 // A cachingRepo is a cache around an underlying Repo,
@@ -287,7 +287,7 @@ func readDiskStat(path, rev string) (file string, info *RevInfo, err error) {
 // just to find out about a commit we already know about
 // (and have cached under its pseudo-version).
 func readDiskStatByHash(path, rev string) (file string, info *RevInfo, err error) {
-	if SrcMod == "" {
+	if PkgMod == "" {
 		// Do not download to current directory.
 		return "", nil, errNotCached
 	}

src/cmd/go/internal/modfetch/fetch.go

@@ -28,9 +28,9 @@ var downloadCache par.Cache
 // local download cache and returns the name of the directory
 // corresponding to the root of the module's file tree.
 func Download(mod module.Version) (dir string, err error) {
-	if SrcMod == "" {
+	if PkgMod == "" {
 		// Do not download to current directory.
-		return "", fmt.Errorf("missing modfetch.SrcMod")
+		return "", fmt.Errorf("missing modfetch.PkgMod")
 	}
 
 	// The par.Cache here avoids duplicate work but also
@@ -53,7 +53,7 @@ func Download(mod module.Version) (dir string, err error) {
 			if _, err := os.Stat(zipfile); err == nil {
 				// Use it.
 				// This should only happen if the mod/cache directory is preinitialized
-				// or if src/mod/path was removed but not src/mod/cache/download.
+				// or if pkg/mod/path was removed but not pkg/mod/cache/download.
 				fmt.Fprintf(os.Stderr, "go: extracting %s %s\n", mod.Path, mod.Version)
 			} else {
 				if err := os.MkdirAll(filepath.Dir(zipfile), 0777); err != nil {
@@ -200,7 +200,7 @@ func readGoSum(file string, data []byte) {
 
 // checkSum checks the given module's checksum.
 func checkSum(mod module.Version) {
-	if SrcMod == "" {
+	if PkgMod == "" {
 		// Do not use current directory.
 		return
 	}
@@ -264,7 +264,7 @@ func checkOneSum(mod module.Version, h string) {
 // Sum returns the checksum for the downloaded copy of the given module,
 // if present in the download cache.
 func Sum(mod module.Version) string {
-	if SrcMod == "" {
+	if PkgMod == "" {
 		// Do not use current directory.
 		return ""
 	}

src/cmd/go/internal/modfetch/proxy.go

@@ -77,10 +77,10 @@ archive.
 
 Even when downloading directly from version control systems,
 the go command synthesizes explicit info, mod, and zip files
-and stores them in its local cache, $GOPATH/src/mod/cache/download,
+and stores them in its local cache, $GOPATH/pkg/mod/cache/download,
 the same as if it had downloaded them directly from a proxy.
 The cache layout is the same as the proxy URL space, so
-serving $GOPATH/src/mod/cache/download at (or copying it to)
+serving $GOPATH/pkg/mod/cache/download at (or copying it to)
 https://example.com/proxy would let other users access those
 cached module versions with GOPROXY=https://example.com/proxy.
 `,

src/cmd/go/internal/modload/help.go

@@ -50,7 +50,7 @@ GOPATH/src and itself contains a go.mod file or is below a directory
 containing a go.mod file.
 
 In module-aware mode, GOPATH no longer defines the meaning of imports
-during a build, but it still stores downloaded dependencies (in GOPATH/src/mod)
+during a build, but it still stores downloaded dependencies (in GOPATH/pkg/mod)
 and installed commands (in GOPATH/bin, unless GOBIN is set).
 
 Defining a module