Add pcdepends rule

This rule checks if dependencies from pkg-config .pc files are satisfied. These are used by the pkg-config --cflags --libs <library name> command to set the appropriate CFLAGS and LDFAGS when building any dependent (direct and indirect) packages. If any of dependencies specified in Requires or Requires.private fields are missing, pkg-config trows an error. These dependencies are mostly covered by link-level dependencies, but there are some exceptions, this is why we need this rule.

Example output:

libxi E: Dependency libxfixes detected and not included (pkg-config files ['usr/lib/pkgconfig/xfixes.pc'] needed in files ['usr/lib/pkgconfig/xi.pc'])
libxi W: Dependency libx11 detected and implicitly satisfied (pkg-config files ['usr/lib/pkgconfig/x11.pc'] needed in files ['usr/lib/pkgconfig/xi.pc'], libraries ['usr/lib/libX11.so.6'] needed in files ['usr/lib/libXi.so.6.1.0'])
libxi W: Dependency xorgproto detected and implicitly satisfied (pkg-config files ['usr/share/pkgconfig/inputproto.pc'] needed in files ['usr/lib/pkgconfig/xi.pc'])
libxi I: Dependency libxext detected and satisfied (pkg-config files ['usr/lib/pkgconfig/xext.pc'] needed in files ['usr/lib/pkgconfig/xi.pc'], libraries ['usr/lib/libXext.so.6'] needed in files ['usr/lib/libXi.so.6.1.0'])

Namcap/rules/__init__.py

@@ -40,6 +40,7 @@ from . import (
   missingbackups,
   pathdepends,
   perllocal,
+  pcdepends,
   permissions,
   py_mtime,
   pydepends,