buildifier: fix uninitialised check when using list unpack assignment

[Gandem]

Fixes https://github.com/bazelbuild/buildtools/issues/1201

As the test added in this commit shows, prior to this commit, the uninitialized check would incorrectly fail when using list unpack assignment, if the same variable is reused later.

This modifies CollectLValues, so that it supports list unpack assignments (in addition to tuple, and normal assignments).

Please let me know if there's anything that needs to be added/modified in this change, thanks! 🙏

[vladmos]

Could you please update the branch? Usually I can do this myself just before merging, but now I likely don't have permissions to do so.

[Gandem]

@vladmos Done, thanks 🙇 !