This was currently only not allowed for imports.

Really import indices should be unique per context, but
even in that case, concurrency should be allowed.

Fixes #1414

Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>

Change-Id: I944357c7b68cd242d19b323a1380c751a7fe49a5
Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/529629
Reviewed-by: Paul Jolly <paul@myitcv.io>