Fix namespaced providers listing in help

[tsloughter]

Currently namespaces aren't included in the help output so a command like erlydtl compile ends up as:

$ ../rebar3/rebar3 help
....
compile         Compile apps .app.src and .erl files.
compile         Compile erlydtl templates.
....

[ferd]

I have an in-progress change to the providers lib that would do:

Several tasks are available:

as              Higher order provider for running multiple tasks in a sequence as a certain profiles.
clean           Remove compiled beam files from apps.
compile         Compile apps .app.src and .erl files.
ct              Run Common Tests
dialyzer        Run the Dialyzer analyzer on the project.
do              Higher order provider for running multiple tasks in a sequence.
eunit           Run eunit tests on project apps.
help            Display a list of tasks or help for a given task or subtask.
new             Create new project from templates.
pkgs            List available packages.
release         Build release of project.
shell           Run shell with project apps and deps in path.
tar             Tar archive of release built of project.
update          Update package index.
upgrade         Upgrade dependency.
version         Print version for rebar and current Erlang.

efene
  compile        efene rebar3 plugin

erlydtl
  compile        Compile erlydtl templates.

Run 'rebar help <TASK>' for details.

Does this look good?

[ferd]

tsloughter/providers#4