deprecate use of Pod::LaTeX from core

These commits should cause Pod::LaTeX to install to "site" in v5.12 and later, and to issue a deprecation warning when run from the core install starting with v5.17.

I have not tested the deprecation warning yet, and am making this pull request tentatively.  Either I will test it and update the PR or someone else will test it.  I just need to switch tasks now.