xbps-src: a path to the sourcepkg directory can now be used.

For example:

	$ ./xbps-src install srcpkgs/foo
	$ ./xbps-src install /path/to/void-packages/srcpkgs/foo
	$ ./xbps-src install foo
	$ cd srcpkgs/foo && xbps-src (if void-packages is in PATH)

The basename of the pkgname argument is now used.