Introduce a way to override dependencies for all pkgs thru a file.

The `etc/defaults.virtual` file contains the default replacement
for our use cases; at this moment we only use it to use `eudev`
rather than `systemd` for udev.

If `etc/virtual` exists it overrides the settings of `etc/defaults.virtual`.
srcpkgs: switch pkgs to depend on libudev again; we use virtual replacements now.