bpo-35936: Updates to modulefinder (GH-11787)

bpo-35936: Updates to modulefinder (GH-11787)

* Properly handle SyntaxErrors in Python source files.

SyntaxErrors in the target module will rise normally, while SyntaxErrors in dependencies will be added to badmodules. This includes a new regression test.

* Fix name collision bug.

This fixes an issue where a "fromlist" import with the same name as a previously failed import would be incorrectly added to badmodules. This includes a new regression test.

* Replace mutable default values.

Bound empty lists have been replaced with the "if param is None" idiom.

* Replace deprecated imp usage.

Constants imported from imp have been moved to private module-level constants, and ModuleFinder.find_module has been refactored to use importlib. Other than an improvement on how frozen builtin imports are reported (as the frozen imports they are, rather than the stdlib modules they *may* have originated from), these changes maintain complete compatibility with past versions... including odd behavior for returning relative (below current directory, but not a C extension) vs. absolute (above current directory, or a C extension) paths.

Patch by Brandt Bucher.