cmd/cue: do not allow embedding of CUE file types (for now)

Per the design proposed in https://cuelang.org/discussion/3264, we do
not allow CUE file type embeds for now. This CL fixes the MVP
implementation to match that initial decision, along with removing the
attempted embed of a CUE file across a module boundary. Preventing
embedding of files across module boundaries is fixed in another CL, but
this change to not attempt the CUE file type embed is needed now, and
the case covered in embed_err.txtar.

Fixes #3258.

Signed-off-by: Paul Jolly <paul@myitcv.io>
Change-Id: Ic12297750620ade0cd631a56f73f16c3d0ca156e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1197337
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cmd/cue/cmd/testdata/script/embed.txtar

@@ -44,7 +44,6 @@ special: {
 	// probably be up to the loader to provide a fs.FS that handles this
 	// according to spec.
 	nestedModJSON: _ @embed(file="a/b/foo.json")
-	nestedModFile: _ @embed(file="a/b/cue.mod/modules.cue")
 }
 
 -- test.json --
@@ -124,7 +123,6 @@ special: {
     underscoreDir:  _
     dotDir:         _
     nestedModJSON:  _
-    nestedModFile:  _
 }
 -- out/eval --
 a: {
@@ -219,12 +217,6 @@ special: {
         a: 1
         b: 2
     }
-    nestedModFile: {
-        module: "acme.com"
-        language: {
-            version: "v0.9.0"
-        }
-    }
 }
 -- out/export --
 a: x:  34
@@ -285,9 +277,5 @@ special: {
 		a: 1
 		b: 2
 	}
-	nestedModFile: {
-		module: "acme.com"
-		language: version: "v0.9.0"
-	}
 }
 -- out/vet --

cmd/cue/cmd/testdata/script/embed_err.txtar

@@ -114,3 +114,7 @@ x: 5
     ./test.cue:39:10
 @embed: open x\test.json: no such file or directory:
     ./test.cue:41:18
+@embed: encoding "cue" not (yet) supported:
+    ./test.cue:43:20
+@embed: encoding "cue" not (yet) supported:
+    ./test.cue:44:24

cue/interpreter/embed/embed.go

@@ -303,6 +303,8 @@ func (c *compiler) decodeFile(file, scope string, schema adt.Value) (adt.Expr, e
 	}
 
 	switch f.Encoding {
+	case build.CUE:
+		return nil, errors.Newf(c.pos, "encoding %q not (yet) supported", f.Encoding)
 	case build.JSONL:
 		return nil, errors.Newf(c.pos, "encoding %q not (yet) supported: requires support for streaming", f.Encoding)
 	case build.BinaryProto, build.TextProto: