builder: Allow git uris to be relative to the base directory
builder: Allow git uris to be relative to the base directory

This allows you to use ".git" as the git uri, thus building from the
current source directory.

matthiasclasen/flatpak,alexlarsson/xdg-app,matthiasclasen/xdg-app,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,alexlarsson/xdg-app,matthiasclasen/xdg-app,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/xdg-app,handsome-feng/flatpak,handsome-feng/flatpak,amigadave/flatpak,matthiasclasen/flatpak,amigadave/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/xdg-app,GeorgesStavracas/flatpak,flatpak/flatpak,alexlarsson/xdg-app,matthiasclasen/flatpak,alexlarsson/xdg-app,GeorgesStavracas/flatpak,flatpak/flatpak,amigadave/flatpak,amigadave/flatpak,matthiasclasen/flatpak