debug('Calculating delta (reset: %s, shallow: %s)', reset, shallow);
    // If there is already a build in progress, wait until it finish to start
    // processing a new one (delta server doesn't support concurrent builds).
    if (this._currentBuildPromise) {
      await this._currentBuildPromise;
    }
Main method to calculate the delta of modules. It returns a DeltaResult,
which contain the modified/added modules and the removed modules.