note that now ALL `src_dirs` across included profiles are
compiled. previously only the last included profile's `src_dirs`
were used

Conflicts:
	src/rebar_state.erl

When fetching dependencies for the first time using a profile (`rebar3
as prod release` or `rebar3 ct`), the dependencies get fetched into the
non-default profile. This has two consequences:

- the files get re-downloaded on follow-up runs
- the lock file includes incomplete or too many deps in its list

This patch forces dependencies in the default profile to be stored in
_build/default/lib even when running under other profiles, then symlinks
them to the correct one.

This makes it so common dependencies in 'default' be downloaded there
and avoids re-downloading them. Should also fix the lock issues.

to `as` used to generate them