lib.licenses: introduce `redistributable` attribute

Allows for distinction of licenses that are unfree overall but do grant the
right to redistribute. Defaults to the freeness of the license.

Note: Many unfree but are redistributable licenses aren't marked as such.
I expect that to be fixed in a distributed manner over time.

Closes https://github.com/NixOS/nixpkgs/pull/97789

lib/licenses.nix

@@ -13,9 +13,11 @@ lib.mapAttrs (lname: lset: let
       if license ? spdxId
       then license // { url = "https://spdx.org/licenses/${license.spdxId}.html"; }
       else license;
+    applyRedistributable = license: { redistributable = license.free; } // license;
   in lib.pipe licenseDeclaration [
     applyDefaults
     applySpdx
+    applyRedistributable
   ];
 in mkLicense lset) ({
   /* License identifiers from spdx.org where possible.