Use a common constant for detecting bzlmod being enabled

[rickeylev]

Various parts of the codebase detect whether bzlmod is enabled or not. Most of them copy/paste the same if @ in Label(..) trick, and use a comment to explain what they're doing.

Rather that copy/paste that everywhere, lets just have a constant defined that does this once, then code can simply check if BZLMOD_ENABLED.

PR #1287 is adding such a constant; this issue is for cleaning up the code to use it everywhere.