[analyzer] Force dependency checkers to be hidden

Since strong dependencies aren't user-facing (its hardly ever legal to disable
them), lets enforce that they are hidden. Modeling checkers that aren't
dependencies are of course not impacted, but there is only so much you can do
against developers shooting themselves in the foot :^)

I also made some changes to the test files, reversing the "test" package for,
well, testing.

Differential Revision: https://reviews.llvm.org/D81761