Avoid "No such file or directory" if module ancestor doesn't exist

[gvanrossum]

Tentative fix for #5784.

(An alternative fix would be not to add anything to self.ns_ancestors unless self.options.namespace_packages is set.)

[gvanrossum]

Beware: When testing manually, the cache may suppress errors.

[gvanrossum]

I am now sufficiently confident that this version of the fix (which leaves all the original code in place but adds an isdir() check before putting a path in ns_ancestors) is right. I am going to merge this and cherry-pick it into the release-0.641 branch.

(Why am I confident? I added a bunch of print() calls and tested this with the repro from #5784 (import aiohttp), as well as with the tests from #5758 (typedpkg_namespace.alpha), and it gives exactly the expected errors and the output explains why.)

However before I re-cut a release I want #5782 (Refactor testpep561 etc.) to land too. That requires further review. I may or may not merge that into the release-0.641 branch (since I like the idea of having absolutely minimal changes in a "brown bag release" like that).