cmd/cue: exp gengotypes emits unhelpful error given a packageless CUE file

[jpluscplusm]

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

$ cue version
cue version v0.13.0-0.dev.0.20250217140725-3545df15d578

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?

! exec cue exp gengotypes file.cue
! stderr 'expected ''IDENT'', found ''type'''
-- file.cue --
#X: {
  y: int
  z: string
}

What did you expect to see?

A passing test, indicating a friendlier error message that told me what I'd done to trigger the error.

What did you see instead?

> ! exec cue exp gengotypes file.cue
[stderr]
-- $WORK/cue_types_gen.go --
   1: // Code generated by "cue exp gengotypes"; DO NOT EDIT.
   2: 
   3: package 
   4: 
   5: type X struct {
   6: Y int64 `json:"y"`
   7: 
   8: Z string `json:"z"`
   9: 
  10: }
  11: 
  12: 

--
5:1: expected 'IDENT', found 'type'
[exit status 1]
> ! stderr 'expected ''IDENT'', found ''type'''
FAIL: cue.exp.gengotypes.packagelessFilesErrorIsUnhelpful.txtar:2: unexpected match for `expected 'IDENT', found 'type'` found in stderr: expected 'IDENT', found 'type'

[mvdan]

My thinking is to make this a nicer error, but still fail, because currently the CUE package name is crucial to know what filename to output to - and also what to name the resulting Go package.