Background

This is another part of introducing pants, as discussed in various TSC meetings.

Related PRs can be found in:

• pantsbuild milestone (which includes PRs since Initialize pantsbuild #5713)
• pantsbuild label (which also includes preparatory PRs that came before adding pantsbuild)

Overview of this PR

This is a grab-bag of minor fixes I've noticed.

• These packages do nothing but re-export symbols, which makes it harder to find where things are defined. So, I replaced imports from them with where the symbols are actually defined.
  I would like to delete these - any objections? Do we need to worry about backwards compat for these internal-only bits?
  • st2common.persistence.reactor
  • st2common.models.db.reactor
• Use a relative dep in contrib/packs/tests/BUILD because it feels cleaner.
• Add comments about how to regenerate the lockfile in requirements-pants.txt and lockfiles/st2-constraints.txt.
• Drop the execute bit on st2common/st2common/cmd/*.py. These have no "main" check, so they're not directly executable. The real executables are in st2common/bin, and those do have the proper execute bit set.
• Correct some copy/pasta comments that mentioned the wrong resource.