Distinguish module qualifiers by sys_info

Summary: Pysa builds a unique module qualifier for every module. When several modules share the same module name, it disambiguates them so each gets a distinct qualifier.

Previously this only disambiguated by source path (the shortest unique suffix of the path). But two modules can share both a name and a source path while differing in sys_info (python version + platform) - for example typeshed builtins analyzed under two different python versions/platforms. In that case the source-path disambiguation cannot tell them apart, so they collapse onto a single qualifier.

This decides each disambiguation dimension independently, adding it only when it actually varies across the same-name group:

• should_add_path_prefix (more than one distinct source path): use the shortest unique suffix of the source path (find_shortest_unique_prefix); otherwise the path component is empty.
• should_add_python_version / should_add_platform (more than one distinct python version / platform): prepend a compact bracketed sys_info element that includes only the varying components - [py<major>.<minor>.<micro>], [<platform>], or [py<major>.<minor>.<micro>-<platform>].

Reviewed By: tianhan0

Differential Revision: D114231139

fbshipit-source-id: d695fc76bad10279fcfdfb7196deec24aaa2184c