setup.py: Rework pycairo discovery to not use pkg-config
Two problems with the previous approach:
* We looked into sys.prefix for the .pc file while the package
could be installed in another location like the user dir location
or on Debian it goes into /usr/local.
* The .pc file intalled by pycairo can contain wrong paths if
installed through pip, since that sometimes builds a wheel
and caches that for future installations to different locations.
Instead of using the .pc file use pkg_resources to discover the
pycairo location and then try to find the correspinding "data" installation
path for the first matching location scheme.
See https://github.com/pygobject/pycairo/issues/85