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.

fixes #331

alongside their source (ie, if `some_tests` is a directory that
contains test suites beams resulting from compiling them will be
placed in `some_tests` in the appropriate `_build` directory

generate line by line coverage reports

- Allow 'rebar3 help' to work and have it point to 'rebar3 help
  <template>'
- show a 'template not found' message when a template is indeed not
  found rather than crashing.