cue: ignore bad symlinks and unusable file types

- Ignore symlinks whose targets cannot be stat'd
- Ignore sockets, devices, etc.

closes cue-lang#1672

Signed-off-by: Kevin Burge <kcburge@pm.me>

cue/load/import.go

@@ -174,7 +174,36 @@ func (l *loader) importPkg(pos token.Pos, p *build.Instance) []*build.Instance {
 				return retErr(errors.Wrapf(err, pos, "import failed reading dir %v", dirs[0][1]))
 			}
 			for _, f := range files {
-				if f.IsDir() {
+				switch mode := f.Mode(); mode & os.ModeType {
+				case 0:
+				case os.ModeDir:
+					continue
+				case os.ModeSymlink:
+					l, err := os.Stat(f.Name())
+					if err != nil {
+						p.UnknownFiles = append(p.UnknownFiles, &build.File{
+							Filename:      f.Name(),
+							ExcludeReason: errors.Newf(token.NoPos, "bad link"),
+						})
+						continue // skip bad symlinks
+					}
+
+					switch lmode := l.Mode(); lmode & os.ModeType {
+					case 0:
+					case os.ModeDir:
+						continue
+					default:
+						p.UnknownFiles = append(p.UnknownFiles, &build.File{
+							Filename:      f.Name(),
+							ExcludeReason: errors.Newf(token.NoPos, "unknown file %d", lmode),
+						})
+					}
+
+				default:
+					p.UnknownFiles = append(p.UnknownFiles, &build.File{
+						Filename:      f.Name(),
+						ExcludeReason: errors.Newf(token.NoPos, "unknown file %d", mode),
+					})
 					continue
 				}
 				if f.Name() == "-" {