because we will expand GAIA_DISTRIBUTION_DIR/<PATH_IN_APPLIST> on https://github.com/yurenju/gaia/blob/25c1301be71b3095249b534d94a210fedd43b609/Makefile#L199-L213, so we no longer need to push it to additionalSrcDirNames.

and I tested to assign GAIA_DISTRIBUTION_DIR for picking apps, it works fine :)