cmd/go: log compilation errors when scanning directories and packages

Before, some packages disappear silently if the package cannot be imported,
such as if the import statement is unparseable.
Before:
        % ls src
        foo   issue
        % go list ./...
        _/home/r/bug/src/foo
        %
After:
        % go list ./...
        src/issue/issue.go:3:5: expected 'STRING', found newline
        _/home/r/bug/src/foo
        %

R=rsc
CC=golang-dev
https://golang.org/cl/10568043

src/cmd/go/main.go

@@ -486,6 +486,9 @@ func matchPackages(pattern string) []string {
 		}
 		_, err = buildContext.ImportDir(path, 0)
 		if err != nil {
+			if _, noGo := err.(*build.NoGoError); !noGo {
+				log.Print(err)
+			}
 			return nil
 		}
 		pkgs = append(pkgs, name)
@@ -520,8 +523,10 @@ func matchPackages(pattern string) []string {
 				return nil
 			}
 			_, err = buildContext.ImportDir(path, 0)
-			if err != nil && strings.Contains(err.Error(), "no Go source files") {
-				return nil
+			if err != nil {
+				if _, noGo := err.(*build.NoGoError); noGo {
+					return nil
+				}
 			}
 			pkgs = append(pkgs, name)
 			return nil
@@ -588,6 +593,9 @@ func matchPackagesInFS(pattern string) []string {
 			return nil
 		}
 		if _, err = build.ImportDir(path, 0); err != nil {
+			if _, noGo := err.(*build.NoGoError); !noGo {
+				log.Print(err)
+			}
 			return nil
 		}
 		pkgs = append(pkgs, name)