Load resource_files from external project

[LecrisUT]

Issue

Right now most of the resources are searched through resource_files, but if other projects want to inject files there, e.g. json schemas, there is no way to make it searchable.

Approach 1

Extend resource_files to allow more general Traversable types:

tmt/tmt/utils.py

Lines 7028 to 7030 in b782a60

	def resource_files(path: Union[str, Path], package: Union[str, ModuleType] = "tmt") -> Path:
	"""
	Helper function to get path of package file or directory.

This is in order to allow the usage of MultiplexedPath, where the roots are package="tmt" and entry-point tmt.resources

Difficulty: For this approach, many implementation must be generalized from Path to Traversable until the final item where it will be cast to Path. For the most part as I navigated, there doesn't seem to be much issue except for tmt.utils.load_schema_store where glob is used. In principle that could be handled by using iterdir instead.

Approach 2

For each resource_files implement a search mechanism to look in each entry-point.

Difficulty:

• How to convert variables like TEMPLATES_DIRECTORY?
• Increased complexity since this search will now be done near each execution of resource_files
• Will not be able to support native MultiplexedPath e.g. from namespaced packages of the third-party project