Rename IndexBuilder -> ShardBuilder (#908)

When navigating the code, I've often forgotten the difference between
`NewBuilder` and `NewIndexBuilder`. This rename clarifies that one of these
indexes a whole repo, while the other builds individual shards. Also
`index.NewShardBuilder` sounds better.

cmd/zoekt-sourcegraph-indexserver/cleanup_test.go

@@ -173,7 +173,7 @@ func createTestShard(t *testing.T, repo string, id uint32, path string, optFns .
 	for _, optFn := range optFns {
 		optFn(r)
 	}
-	b, err := index.NewIndexBuilder(r)
+	b, err := index.NewShardBuilder(r)
 	if err != nil {
 		t.Fatal(err)
 	}

index/builder.go

@@ -1020,13 +1020,13 @@ func (b *Builder) CheckMemoryUsage() {
 	}
 }
 
-func (b *Builder) newShardBuilder() (*IndexBuilder, error) {
+func (b *Builder) newShardBuilder() (*ShardBuilder, error) {
 	desc := b.opts.RepositoryDescription
 	desc.HasSymbols = !b.opts.DisableCTags && b.opts.CTagsPath != ""
 	desc.SubRepoMap = b.opts.SubRepositories
 	desc.IndexOptions = b.opts.GetHash()
 
-	shardBuilder, err := NewIndexBuilder(&desc)
+	shardBuilder, err := NewShardBuilder(&desc)
 	if err != nil {
 		return nil, err
 	}
@@ -1035,7 +1035,7 @@ func (b *Builder) newShardBuilder() (*IndexBuilder, error) {
 	return shardBuilder, nil
 }
 
-func (b *Builder) writeShard(fn string, ib *IndexBuilder) (*finishedShard, error) {
+func (b *Builder) writeShard(fn string, ib *ShardBuilder) (*finishedShard, error) {
 	dir := filepath.Dir(fn)
 	if err := os.MkdirAll(dir, 0o700); err != nil {
 		return nil, err

index/eval_test.go

@@ -202,7 +202,7 @@ func compoundReposShard(t *testing.T, names ...string) *indexData {
 		docs = append(docs, ds)
 	}
 
-	b := testIndexBuilderCompound(t, repos, docs)
+	b := testShardBuilderCompound(t, repos, docs)
 	s := searcherForTest(t, b)
 	return s.(*indexData)
 }
@@ -375,7 +375,7 @@ func hash(name string) uint32 {
 
 func TestGatherBranches(t *testing.T) {
 	content := []byte("dummy")
-	b := testIndexBuilder(t, &zoekt.Repository{
+	b := testShardBuilder(t, &zoekt.Repository{
 		Branches: []zoekt.RepositoryBranch{
 			{"foo", "v1"},
 			{"foo-2", "v1"},