cmd/cue: exp gengotypes output filenames vary based on CUE module presence

[jpluscplusm]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.13.0-0.dev.0.20250217164507-bb1be011fa01

go version go1.23.5
      -buildmode exe
       -compiler gc
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v1
cue.lang.version v0.13.0

Does this issue reproduce with the latest stable release?

Yes, v0.12.0.

What did you do?

# Inside a CUE module.
exec cue mod init
exec cue exp gengotypes .:foo
exec find . -type f -name *.go
cmp stdout files

# Reset state.
rm -rf cue.mod
rm cue_types_foo_gen.go

# Not inside a CUE module.
exec cue exp gengotypes .:foo
exec find . -type f -name *.go
cmp stdout files

-- file.cue --
package foo

#X: int
-- files --
./cue_types_foo_gen.go

What did you expect to see?

A passing test indicating that, when invoked outside a CUE module, cue exp gogentypes emitted a Go file whose name encoded the CUE package name for which it was being called.

What did you see instead?

# Inside a CUE module. (0.042s)
# Reset state. (0.000s)
# Not inside a CUE module. (0.018s)
> exec cue exp gengotypes .:foo
> exec find . -type f -name *.go
[stdout]
./cue_types_gen.go
> cmp stdout files
diff stdout files
--- stdout
+++ files
@@ -1,1 +1,1 @@
-./cue_types_gen.go
+./cue_types_foo_gen.go

FAIL: cue.exp.gengotypes.outFilenameVaries.txtar:14: stdout and files differ