Do not record or check repo mapping entries for repos defined in WORKSPACE

[Wyverald]

Mappings for repos defined in WORKSPACE are extremely annoying to verify given the chunked loading (we'd have to record which chunk the repo mapping was used in, and then load that chunk while verifying). This is extremely not worth the effort (especially since nobody really uses repo mappings in WORKSPACE) so we just don't record it.

This also means, during verification, we can safely use the main repo mapping without WORKSPACE (since repos defined in Bzlmod can't see stuff from WORKSPACE anyway).

Fixes #21289.

[Wyverald]

cc @rrbutani @meteorcloudy @SalmaSamy

[rrbutani]

I can confirm that this fixes the issue for me (in the repro in #21289 and in the actual workspace that it was extracted from). @Wyverald thank you for the speedy fix! :smile:

[lberki]

@Wyverald mind assigning this review to someone else? I'd have to get up to speed on the intricacies of module extensions etc. to review this change (I should do that eventually anyways and I can do that now if there is no other option). Maybe @SalmaSamy ?