[release-branch.go1.11] go/packages: make tests pass with custom GOCACHE

This commit was merged earlier with some failing trybots, so it was
reverted. This is a re-submission.

Before this change, a test would fail:

	$ GOCACHE=$HOME/go/cache go test
	--- FAIL: TestLoadImportsGraph (1.05s)
	    packages_test.go:225: subdir/d.test.Srcs = [cf570d60b25cde4f49bbe5f69d3ed407f2d7f1fbc500b8807da726fb19b8f588-d], want [0.go]
	FAIL

This is because it assumed that the user hadn't set their own GOCACHE,
and thus that all source files in the cache would be under the default
"go-build" cache directory.

We could fix this via os.Getenv("GOCACHE"), but a simpler mechanism is
to see if the source file has an extension. Source files don't have an
extension in GOCACHE, so that's much simpler to detect.

After this change:

	$ GOCACHE=$HOME/go/cache go test
	PASS

On release-branch.go1.11, golist_fallback.go did not yet have the code
added that would need to be fixed, so nothing is being backported to it
in this change.

While at it, gofmt.

Updates golang/go#29445
Fixes golang/go#29944

Change-Id: I21fc59f13f00bea1f9a8a80e0438825f1a36ac3e
Reviewed-on: https://go-review.googlesource.com/c/156977
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/163780
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/go/packages/packages_test.go b/go/packages/packages_test.go
index 3ee4e87..b3a78aa 100644
--- a/go/packages/packages_test.go
+++ b/go/packages/packages_test.go
@@ -1207,19 +1207,19 @@
 		ID:   "b",
 		Name: "b",
 		Imports: map[string]*packages.Package{
-			"a": &packages.Package{ID: "a"},
+			"a": {ID: "a"},
 		},
 	}, {
 		ID:   "c",
 		Name: "c",
 		Imports: map[string]*packages.Package{
-			"b": &packages.Package{ID: "b"},
+			"b": {ID: "b"},
 		},
 	}, {
 		ID:   "d",
 		Name: "d",
 		Imports: map[string]*packages.Package{
-			"b": &packages.Package{ID: "b"},
+			"b": {ID: "b"},
 		},
 	}} {
 		got := decoded[i]
@@ -1267,12 +1267,13 @@
 func cleanPaths(paths []string) []string {
 	result := make([]string, len(paths))
 	for i, src := range paths {
-		// The default location for cache data is a subdirectory named go-build
-		// in the standard user cache directory for the current operating system.
-		if strings.Contains(filepath.ToSlash(src), "/go-build/") {
+		// If the source file doesn't have an extension like .go or .s,
+		// it comes from GOCACHE. The names there aren't predictable.
+		name := filepath.Base(src)
+		if !strings.Contains(name, ".") {
 			result[i] = fmt.Sprintf("%d.go", i) // make cache names predictable
 		} else {
-			result[i] = filepath.Base(src)
+			result[i] = name
 		}
 	}
 	return result