Support main repository prefixing in cquery

This makes mixed builds resilient to a recent backward-incompatbile
Bazel feature which explicitly adds the main repository (`@`) prefix to
all labels which are stringified by `str(target.label)` in Starlark.

This implementation is compatible with both current Bazel and
Bazel@HEAD.

After a new Bazel release to AOSP, we can clean up this code a little to
only support Bazel with this feature active (and that we need not hedge
on either Bazel behavior).

Bug: 248106697
Test: `m --bazel-mode nothing` with both current Bazel and a new Bazel
Change-Id: Id53c8505bb9080d5073c844de7f1ee57ceceae46