[pkg] Catch exceptions where dependency resolution gets invalid imports (#58573)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58573
Users can create invalid imports, like:
```
HG: in a top-level package
if False:
from .. import foo
```
Since this code is never executed, it will not cause the module to fail to
load. But our dependency analysis walks every `import` statement in the AST,
and will attempt to resolve the (incorrectly formed) import, throwing an exception.
For posterity, the code that triggered this: https://git.io/JsCgM
Differential Revision: D28543980
Test Plan: Added a unit test
Reviewed By: Chillee
Pulled By: suo
fbshipit-source-id: 03b7e274633945b186500fab6f974973ef8c7c7d
