internal/filetypes: split ParseFile

Exposing ParseFileAndType. For the embedding
proposal, file and type are specfied separately
and do not need to be parsed as on the
command line.

Issue #2031

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: Ib02f845d503edf1d78834a1ff2a0c224cc936748
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1196716
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

internal/filetypes/filetypes.go

@@ -260,6 +260,12 @@ func ParseFile(s string, mode Mode) (*build.File, error) {
 		}
 		return nil, errors.Newf(token.NoPos, "empty file name")
 	}
+
+	return ParseFileAndType(file, scope, mode)
+}
+
+// ParseFileAndType parses a file and type combo.
+func ParseFileAndType(file, scope string, mode Mode) (*build.File, error) {
 	// Quickly discard files which we aren't interested in.
 	// These cases are very common when loading `./...` in a large repository.
 	typesInit()