cue/load: don't hang on files named -

Note that the check is quite carefully placed to still allow
`cue eval -` to select stdin, even if the current directory
has a file named '-'.

Fixes #422

Change-Id: Ic60fa137f929a1102753f1f20a8c7a27728f121c
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7263
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cmd/cue/cmd/testdata/script/no_read_stdin.txt

@@ -0,0 +1,9 @@
+# try to trick cue reading from a perpetually open stdin by placing a file
+# named '-' in the current directory.
+
+! cue eval
+stdin -
+cmp stderr expect-stderr
+-- - --
+-- expect-stderr --
+no CUE files in .

cue/load/import.go

@@ -150,6 +150,11 @@ func (l *loader) importPkg(pos token.Pos, p *build.Instance) *build.Instance {
 				if f.IsDir() {
 					continue
 				}
+				if f.Name() == "-" {
+					if _, err := cfg.fileSystem.stat("-"); !os.IsNotExist(err) {
+						continue
+					}
+				}
 				file, err := filetypes.ParseFile(f.Name(), filetypes.Input)
 				if err != nil {
 					p.UnknownFiles = append(p.UnknownFiles, &build.File{