Sysrepo/libyang detection improvements

#1077 (closed) and !1270 (merged) did a good job replacing our own half-baked sysrepo_config with a standard pkg-config. With it, two of three modes work:

1. --with-libyang --with-sysrepo works
2. --with-libyang=/usr/local --with-sysrepo=/usr/local
3. --with-libyang=/usr/local/lib/pkgconfig/libyang.pc --with-sysrepo=/usr/local/lib/pkgconfig/sysrepo.pc does not

The code doesn't work with --with-libyang=/usr/local/lib/pkgconfig/libyang.pc --with-sysrepo=/usr/local/lib/pkgconfig/sysrepo.pc, even though the --with-libyang and --with-sysrepo help suggests it should. We need to rethink whether this use case really makes sense as there are really 4 .pc files: libyang.pc, libyang-cpp.pc, sysrepo.pc, and sysrepo-cpp.pc.

I think it's fine to get rid of this mode, so only 1. and 2. be supported.