lib.meta: Minor SPDX license function improvements

- Expose `lib.licensesSpdx`
- Create bindings for the needed internal functions
- Mention that some SPDX licenses might be missing (in the future I hope
  we can autogenerate the Nixpkgs license list from some SPDX endpoint

lib/default.nix

@@ -123,7 +123,8 @@ let
     inherit (self.derivations) lazyDerivation optionalDrvAttr;
     inherit (self.meta) addMetaAttrs dontDistribute setName updateName
       appendToName mapDerivationAttrset setPrio lowPrio lowPrioSet hiPrio
-      hiPrioSet getLicenseFromSpdxId getLicenseFromSpdxIdOr getExe getExe';
+      hiPrioSet licensesSpdx getLicenseFromSpdxId getLicenseFromSpdxIdOr
+      getExe getExe';
     inherit (self.filesystem) pathType pathIsDirectory pathIsRegularFile
       packagesFromDirectoryRecursive;
     inherit (self.sources) cleanSourceFilter