Merge `pack` into `gvc`

The pack library was being built as an internal library meant to be merged into the gvc library. This migrates the sources into the same directory. It takes the opportunity to remove the libpack.pc.in file which was no longer being processed to generate libpack.pc and no such pkg-config file was being installed. The man page for gvpack is also updated to remove the reference to the libpack entry.

The last remnants of the libpack are in the installed man page for pack (now from the gvc directory). The entity is now fully merged into GVC.

This is not an ABI break, the library has been part of gvc, it simply brings the build systems more inline with each other.