Leo now imports real-world repos perfectly

[Edward K. Ream]

PR #4423, now in the devel branch, is a milestone in Leo's history. For the first time, all of Leo's importers should preserve imported files exactly. Writing @clean nodes back to the file system will not change any of those files!

I have tested this round-trip requirement on two real-world repos:

- the large  pylint sources (including astroid) repos.

- the huge rust compiler sources.

Round-tripping should work for all of Leo's importers because the crucial changes occurred in the base Importer class. And despite the PR's apparent complexity, no part of the fundamental algorithms (in the Importer class) changed!

Summary

This is a great day for Leo. This PR is the culmination of over a decade of work on Leo's importers.

Edward