Add option to skip release discovery

`rlx_prv_discover` provider recursively scans all directories in
`lib_dirs` list looking for `*.app` and `*.rel` files.

This commit adds an option `disable_rel_discovery` to skip searching for
`*.rel` files when we know there aren't any. This speeds up release
generation with a lot of dependencies on the `libs_dir` list
significantly and reduces total release generation time in half.
Accelerate `rlx_prv_discover` provider

Searching for `*.app` files is now done using `filelib:wildcard/1` and
doesn't recursively traverse all directories under `libs_dir` root dirs.

Instead we assume that each directory on `libs_dir` list contains a flat
list of application dirs.