cmd/cue: regression with CUE_EXPERIMENT=modules where unnamed files are also loaded

[mvdan]

$ cue version
cue version v0.9.0-alpha.4.0.20240514085722-e9bf33c50d3f

go version devel go1.23-7f9edb4225 2024-05-11 20:38:24 +0000
      -buildmode exe
       -compiler gc
        -ldflags -w -s
  DefaultGODEBUG asynctimerchan=1,gotypesalias=0,httplaxcontentlength=1,httpmuxgo121=1,tls10server=1,tlsrsakex=1,tlsunsafeekm=1,winreadlinkvolume=0,winsymlink=0
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v1
cue.lang.version v0.9.0
$ cat repro-cmd.txtar 
exec cue export valid.cue
cmp stdout stdout.golden

env CUE_EXPERIMENT=modules

exec cue export valid.cue
cmp stdout stdout.golden

-- stdout.golden --
{
    "out": "bar"
}
-- cue.mod/module.cue --
module: "foo.test/bar"
language: version: "v0.9.0"
-- valid.cue --
#foo: "bar"
out: #foo
-- invalid.cue --
foo :: "bar"
out: foo
$ testscript -v repro-cmd.txtar 
[...]
> exec cue export valid.cue
[stdout]
{
    "out": "bar"
}
> cmp stdout stdout.golden
> env CUE_EXPERIMENT=modules
> exec cue export valid.cue
[stderr]
instance: cannot enumerate all module imports: cannot read "invalid.cue": expected operand, found ':'
[exit status 1]
FAIL: /tmp/testscript2634378284/repro-cmd.txtar/script.txtar:6: unexpected command failure
error running repro-cmd.txtar in /tmp/testscript2634378284/repro-cmd.txtar

This appears to be some sort of bug in cmd/cue or cue/load - when naming a single file to load, we shouldn't be parsing other files in the same directory at all. This regression was spotted via Unity.