internal/filetypes: unexpected acceptance of arbitrary tag=value pairs

[rogpeppe]

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

$ cue version
v0.10.0-alpha.1

Does this issue reproduce with the latest stable release?

Yes (v0.9.2)

What did you do?

! exec cue export json+iddfghdfg=sdfvdfsvb+anythingelse=blah: x

-- x --
{"foo": true}

What did you expect to see?

A passing test

What did you see instead?

> ! exec cue export json+unknowntag=somearbitraryvalue+anythingelse=blah: x
[stdout]
{
    "foo": true
}
FAIL: /tmp/x.txtar:1: unexpected command success

Even though there is clearly no tag unknowntag pr anythingelse, the filetype logic lets it through without issue.

There is probably further conversation to be had about whether attr=val-style filetype attributes
are worth having in general. The ability to have arbitrary values arguably makes it harder to
solve issue #3280 because the set of possible parameters becomes open-ended,
and this style of attribute is not actually used for anything significant in practiced currently.

[rogpeppe]

Fixed by https://cuelang.org/cl/1200901