makepkg: rework libprovides

makepkg: rework libprovides

This reworks the implmentation of libprovides for the following
benefits:
 - Moves functionality from write_pkginfo() to find_libprovides()
 - Only calculates the version for libraries specifically requested
   and not all libraries. This has the disadvantage of running find
   over the $pkgdir for as many libraries as needed, but is unlikely
   to be an issue due to caching.
 - The order of the provides array in the PKGBUILD is kept in the
   package
 - There are more warning messages when things fail and those that
   were there are no longer errors (as I do not think failure of
   libprovides should result in complete packaging failure)
 - It is now modular so can be easy extended to other library types
   other than ELF *.so.

Signed-off-by: Allan McRae <allan@archlinux.org>
Signed-off-by: Dan McGee <dan@archlinux.org>