gopls/internal/lsp/cache: clean up view workspace information

Tracking of workspace information in the View contained several
inconsistencies and redundancies. Clean this up, with the following
changes:

- eliminate the View.rootURI, opting to derive it instead
- eliminate the View.explicitGowork field, instead using the view.gowork
  field and checking if it is outside of the workspace.
- eliminate many places where directory filters were interpreted
  relative to the rootURI. This is wrong: directory filters are
  expressed relative to the workspace folder.
- remove special handling of GOMODCACHE, now that we're on Go 1.16+
- rewrite the locateTemplateFiles function to use view.filterFunc and
  filepath.WalkDir (now that we're on Go 1.16+).
- don't request goimports env vars when loading environment variables
  for the view. They weren't being propagated to goimports anyway, and
  goimports will load them as needed.

For golang/go#55331

Change-Id: I5e7f7e77e86d9ae425d2feaff31030278fed8240
Reviewed-on: https://go-review.googlesource.com/c/tools/+/459789
Reviewed-by: Alan Donovan <adonovan@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

internal/imports/fix.go

@@ -711,7 +711,7 @@ type PackageExport struct {
 	Exports []string
 }
 
-var RequiredGoEnvVars = []string{"GO111MODULE", "GOFLAGS", "GOINSECURE", "GOMOD", "GOMODCACHE", "GONOPROXY", "GONOSUMDB", "GOPATH", "GOPROXY", "GOROOT", "GOSUMDB", "GOWORK"}
+var requiredGoEnvVars = []string{"GO111MODULE", "GOFLAGS", "GOINSECURE", "GOMOD", "GOMODCACHE", "GONOPROXY", "GONOSUMDB", "GOPATH", "GOPROXY", "GOROOT", "GOSUMDB", "GOWORK"}
 
 // ProcessEnv contains environment variables and settings that affect the use of
 // the go command, the go/build package, etc.
@@ -781,7 +781,7 @@ func (e *ProcessEnv) init() error {
 	}
 
 	foundAllRequired := true
-	for _, k := range RequiredGoEnvVars {
+	for _, k := range requiredGoEnvVars {
 		if _, ok := e.Env[k]; !ok {
 			foundAllRequired = false
 			break
@@ -797,7 +797,7 @@ func (e *ProcessEnv) init() error {
 	}
 
 	goEnv := map[string]string{}
-	stdout, err := e.invokeGo(context.TODO(), "env", append([]string{"-json"}, RequiredGoEnvVars...)...)
+	stdout, err := e.invokeGo(context.TODO(), "env", append([]string{"-json"}, requiredGoEnvVars...)...)
 	if err != nil {
 		return err
 	}