Don't try to parse directories named *.go

fillPackage is supposed to use the AST parser on individual files.
Unforntunately, `filepath.Glob("*.go")` returns directories as well as
files.

Fixes golang#550

internal/gps/pkgtree/pkgtree.go

@@ -203,6 +203,12 @@ func fillPackage(p *build.Package) error {
 		if filepath.Base(file)[0] == '_' {
 			continue
 		}
+
+		// Skip any directories that happen to end with ".go"
+		if stat, err := os.Stat(file); err == nil && stat.IsDir() {
+			continue
+		}
+
 		pf, err := parser.ParseFile(token.NewFileSet(), file, nil, parser.ImportsOnly|parser.ParseComments)
 		if err != nil {
 			if os.IsPermission(err) {

internal/gps/pkgtree/pkgtree_test.go

@@ -1280,6 +1280,11 @@ func TestListPackages(t *testing.T) {
 				},
 			},
 		},
+		"skip '.go' directories": {
+			fileRoot:   j("dotgodir"),
+			importRoot: "dotgodir",
+			err:        nil,
+		},
 	}
 
 	for name, fix := range table {