cmd/cue: export complains when a CUE package, a non-CUE schema, and a data file are combined

[jpluscplusm]

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

$ cue version
cue version v0.9.2

go version go1.22.4
      -buildmode exe
       -compiler gc
       -trimpath true
     CGO_ENABLED 0
          GOARCH amd64
            GOOS linux
         GOAMD64 v1
cue.lang.version v0.9.2

Does this issue reproduce with the latest stable release?

v0.9.2 is latest.

What did you do?

# 1
exec cue export package.cue
cmp stdout gold

# 2
exec cue export package.cue schema.json
cmp stdout gold

# 3
exec cue export package.cue schema.json data.yml
cmp stdout gold

# 4
exec cue export .:a
cmp stdout gold

# 5
exec cue export .:a schema.json
cmp stdout gold

# 6
exec cue export .:a schema.json data.yml
cmp stdout gold
-- package.cue --
package a

x: "foo"
-- schema.json --
{ "$schema": "http://json-schema.org/draft-07/schema#", "type": "object",
  "properties": { "x": { "type": "string", "minLength": 1 } } }
-- data.yml --
x: "foo"
-- gold --
{
    "x": "foo"
}

What did you expect to see?

Ideally: a passing test (because there's no meaningful difference (to the consumer!) between command #3 and command #6)

Failing that: a better error message (because the addition of the data file seems unrelated to the error message, which feels like it should have been triggered by command #5, if it were true)

What did you see instead?

# 1 (0.026s)
# 2 (0.049s)
# 3 (0.039s)
# 4 (0.051s)
# 5 (0.063s)
# 6 (0.056s)
> exec cue export .:a schema.json data.yml
[stderr]
cannot combine packages with individual schema files
[exit status 1]
FAIL: /tmp/testscript538769625/cue.export.cannotCombinePackageWithSchemaFile.txtar/script.txtar:22: unexpected command failure