Revert "profiles/package.mask: converted to a directory"

This reverts commit 06a8f9f35240bcdc2208137030fe4cd43d16e67e.

paludis hates such layouts:
  * When adding profile configuration file '/gentoo-32k/gentoo-haskell/profiles/package.mask':
  * In file '/gentoo-32k/gentoo-haskell/profiles/package.mask': Error reading file: 'Error reading from fd 3: Is a directory' (paludis::SafeIFStreamError) (paludis::ConfigFileError)