go/internal/gcimporter: interpret relative srcDir relative to cwd

1) go/types.dir: Correctly return "." if there is no path.
2) go/internal/gcimporter.FindPkg: work-around for build.Import
   (build.Import doesn't produce expected result if srcDir is
   relative). See also issue 14282.

Fixes #14215.

Change-Id: Ia3721f9ad8a1115d2595fe99b04baaf30d5765f2
Reviewed-on: https://go-review.googlesource.com/19393Reviewed-by: Russ Cox <rsc@golang.org>

src/go/internal/gcimporter/gcimporter.go

@@ -31,7 +31,8 @@ var pkgExts = [...]string{".a", ".o"}
 
 // FindPkg returns the filename and unique package id for an import
 // path based on package information provided by build.Import (using
-// the build.Default build.Context).
+// the build.Default build.Context). A relative srcDir is interpreted
+// relative to the current working directory.
 // If no file was found, an empty filename is returned.
 //
 func FindPkg(path, srcDir string) (filename, id string) {
@@ -44,6 +45,9 @@ func FindPkg(path, srcDir string) (filename, id string) {
 	default:
 		// "x" -> "$GOPATH/pkg/$GOOS_$GOARCH/x.ext", "x"
 		// Don't require the source files to be present.
+		if abs, err := filepath.Abs(srcDir); err == nil { // see issue 14282
+			srcDir = abs
+		}
 		bp, _ := build.Import(path, srcDir, build.FindOnly|build.AllowBinary)
 		if bp.PkgObj == "" {
 			return

src/go/types/resolver.go

@@ -483,11 +483,9 @@ func pkgName(path string) string {
 // (Per the go/build package dependency tests, we cannot import
 // path/filepath and simply use filepath.Dir.)
 func dir(path string) string {
-	if i := strings.LastIndexAny(path, "/\\"); i >= 0 {
-		path = path[:i]
+	if i := strings.LastIndexAny(path, `/\`); i > 0 {
+		return path[:i]
 	}
-	if path == "" {
-		path = "."
-	}
-	return path
+	// i <= 0
+	return "."
 }