Use babelgene package to re-implement convertIdentifiers()

[lianos]

We previously included species and identifier maps in this package for this function to work, but since {babelgene} (which powers {msigdbr}) is now a dependency, let's use that.

Note the original function was called remap_identifiers, but since we're using camel case here, we're going with convertIdentifiers (and "converting" is more what we are doing, not remaping (I guess? ... who knows ...))