xlint: add lint for `${pkgname}` in subpackage's `depends` (should use `${sourcepkg}` instead)

[triallax]

People can unwittingly add ${pkgname} to a subpackage's depends, which will cause the subpackage to depend on itself, when instead they actually want the subpackage to depend on the main package. See https://github.com/void-linux/void-packages/pull/44618/files for example.