Returns the tag name of the given library in the given contrib repository, if installed.
Returns false if not installed.
@param {String} repo_name
@param {String} library_name
@return {String|false}
async function(repo_name, library_name) {
      let library = (await this.getContribData()).libraries.find(lib => lib.repo_name === repo_name && lib.library_name === library_name);
      return library ? library.repo_tag : false;
    }