Linking message will only be printed the first time the link or copy
actually happens.

Note:

rebar_file_utils:symlink_or_copy will now return 'exists'
instead of ok, if it did nothing because the link exists.

Nothing was checking the return value yet, so seemed reasonable.

- Crashes in providers lib when no providers in a namespace are bare
- Making sure bareness matches semantics; i.e. a bare provider is
  visible, a non-bare provider is hidden.

They can be culled and reused in one sort pass.

Should fix #360

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.

The option {deps_error_on_conflict, true} will make it so conflicts in
deps being fetched interrupts the operation rather than just display a
warning.

Defaults to `false'.