ModuleDirImport silently fails to find distribution metadata #175
Description
In the pex bootstrap code, the ModuleDirImport class is able to act as a meta path finder owing to its implementation of find_distributions. Unfortunately it silently fails to locate any distribution metadata unless importlib-metadata happens to be included as a third-party dependency in the pex file:
importlib.metadata has been part of the Python standard library since 3.8, which is the minimum version python-rules now supports anyway, so we can probably just solve this by unconditionally importing importlib.metadata instead.