libpkgconf: pkg: fix computation of pkgconf_pkg_t.id on Windows. Windows allows both \ and / as valid path characters. A computed path such as C:\development\libfoo\pkgconfig/foo.pc will result in a computed pkgconf_pkg_t.id of "pkgconfig/foo". Accordingly, correct the path normalization for checking for / after the \ path has been dealt with in all cases.