[Feature] `PackageMatrix.yml`: Support packages in sub-directories.

[Javagedes]

PackageMatrix.yml only searches the workspace root for packages when it is generating the matrix to prevent accidently discovering packages in submodules. Some repositories, such as mu_tiano_platforms may have packages in sub-directories, which are then missed.

The current suggestion is to add an extra-dirs argument to provide additional directories to search for packages in.

The current workaround is to manually specify the packages that are missed using the package-config argument.