Consider how to relativize target Snapshots to source roots

[stuhood]

In #7696, @Eric-Arellano noticed that the sources inputted to the pytest environment have not be relativized to their source roots (which is what allows for absolute imports of python modules).

This leads to a question: should target's source Snapshots be relativized to their source roots by default? If not, what API should we expose to accomplish this?

In v1, a consumer of a Target's sources uses

pants/src/python/pants/build_graph/target.py

Lines 534 to 542 in 5eed2e7

	def sources_relative_to_source_root(self):
	"""
	:API: public
	"""
	if self.has_sources():
	abs_source_root = os.path.join(get_buildroot(), self.target_base)
	for source in self.sources_relative_to_buildroot():
	abs_source = os.path.join(get_buildroot(), source)
	yield os.path.relpath(abs_source, abs_source_root)

to get sources relative to the Target's source root. In v2, a target's Snapshot would need to be cloned into a new relativized version.

[stuhood]

I expect that this choice boils down to: "across various languages, is it more common to need to have target sources be relative to their source root, or to the build root?". After answering that question, we could add either an API to prefix paths to Snapshots, or an API to do the equivalent of a batch relpath.

[stuhood]

Implemented in #7699! Thank you @illicitonion.