hooks/artifacts are always run/resolved from an app unless at top of umbrella

[tsloughter]

All hooks will be run as part of the app_info set of hooks, in compile for example: https://github.com/rebar/rebar3/blob/master/src/rebar_prv_compile.erl#L87 and artifacts will be found relative to an app's output directory, except in the case of being defined in the rebar.config at the top level of an umbrella project. In that case artifacts will be resolved from profile_dir (eg _build/default/) and hooks will be run from the top level before any part of a provider is run, like https://github.com/rebar/rebar3/blob/master/src/rebar_prv_compile.erl#L49

Additionally 3 variable can be used in the artifacts string: base_dir, profile_dir or out_dir -- out_dir is the default path to start finding artifacts but I figured it would be nice to let it be explicit as well, example: {artifacts, ["{{out_dir}}/priv/eleveldb.so"]}.