Don't attach subprofiles to metadata map.

Whenever composite profiles were added to project maps, their children
were also attached as active profiles. This confuses `with-profile` and
profile merging task where excluded profiles only remove the name of the
composite profile, not the subprofiles. This allows uberjar to exclude
the default profile without excluding subprofiles, without excluding
active subprofiles themselves.

So these task actions now produce sensible results:
`lein with-profile +dev uberjar` -> uberjar with dev-dependencies, but
without the other default profiles
`lein -U uberjar` -> uberjar with snapshot updates, but not with any
default dependencies added.

Fixes #1233.