Clean up exporters, specifically those we have for pkg-config

Clean up exporters, specifically those we have for pkg-config

The pkg-config exporters were a special hack, all in
Configurations/unix-Makefile.tmpl, and this was well and good as long
as that was the only main package interface configuration system that we
cared about.

Things have changed, though, so we move the pkg-config production to be
templatable in a more flexible manner.  Additional templates for other
interface configuration systems can then be added fairly easily.

Two variants of the .pc files are produced:

- Those in 'exporters/' are installed in the location that 'pkg-config'
  itself prefers for installed packages.
- Those in the top directory are to be used when it's desirable to build
  directly against an OpenSSL build tree.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20878)