Accelerate `rlx_prv_discover` provider

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 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.