BREAKING CHANGE
This commit modifies the overlay builder's original behavior
in a non-backward compatible way. Previously, all overlays
would be namespaced with "nixpkgs/". This was often
unnecessary or would cause conflicts with package overlays if
they shared a name. After this change, all overlays retain their
directory name (eg. "my-overlay") and all package overlays are
prefixed with "package/" to differentiate them and avoid name
conflicts.
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