cmd/cue: fix should be able to process CUE from stdin

[jpluscplusm]

Is your feature request related to a problem? Please describe.

I have processes that need to operate inside an isolated container, consuming stdin and communicating via stdout.
One of the processes I'd like to run in this way is cue fix - but I can't, as it only consumes named packages (or files?) it can find via the filesytem.

Describe the solution you'd like

I'd like cue fix to accept CUE that needs fixing on stdin, and emit the fixed result to stdout.
I'd like this test to pass:

cp orig.cue unfixed.cue
exec cue fix unfixed.cue
cmp unfixed.cue fixed.cue

stdin orig.cue
exec cue fix -
cmp stdout fixed.cue
-- orig.cue --
x: 5 mod 3
-- fixed.cue --
x: __mod(5, 3)

Describe alternatives you've considered

n/a.

Additional context

n/a

[mvdan]

Here is a version of your testscript without needing a shell:

cp orig.cue unfixed.cue
exec cue fix unfixed.cue
cmp unfixed.cue fixed.cue

stdin orig.cue
exec cue fix -
cmp stdout fixed.cue
-- orig.cue --
x: 5 mod 3
-- fixed.cue --
x: __mod(5, 3)

cue fix - indeed seems to be a no-op:

> stdin orig.cue
> exec cue fix -
> cmp stdout fixed.cue
diff stdout fixed.cue
--- stdout
+++ fixed.cue
@@ -0,0 +1,1 @@
+x: __mod(5, 3)

This could be related to #3405.

[jpluscplusm]

Here is a version of your testscript without needing a shell:

Thanks - I didn't know stdin could work like that! I've edited my original comment to include your updated example.