PkgC: Correctly handle various pkg-config edge cases

Fixes many bugs in PkgC by incorporating the 0.0.7 changes from llpkgc.
These include:
  * Variable substitution fixes
  * Better whitespace handling
  * Correct handling of single and double quotes
  * Dedicated parsers for fragment and dependency lists

Also adds support for pcfiledir, which was blocked on incorrect
whitespace handling in the old parser.

Fixes: #27652, #27683