cmd/cue/cmd: fix data race in import
cmd/cue/cmd: fix data race in import

determining filename uniqueness within directory has
to be done in critical section, of course.

Change-Id: I88ca126d6b5f58d5c1300cb0ad2190864e5cbf23
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/1803
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@google.com>

cuelang/cue