Accelerate application discovery

Introduce option `enable_shallow_app_discovery` which is disabled by default.

When enabled, searching for `*.app` files is done by using `filelib:wildcard/1` and it doesn't recursively traverse all directories under `libs_dir` root dirs.

Instead its assumed that each directory on `libs_dir` list contains a flat list of application dirs or is an app itself.

This pull request should solve #24 and together with #96 gives a lot faster build times which is especially helpful with big number of dependencies.

Our codebase currently has over 70 deps and we were able to reduce release generation times from 25 to 5 seconds.