Allow configuring rename rules for external modules

[emdoyle]

Given cases like #414 , we cannot make the assumption that metadata resolution will always find the appropriate package for a module name.

This PR provides an 'escape hatch' in external.rename, a configured list of rename rules to fix external module resolution.

For example, if your code uses pillow through the PIL module, but check-external is failing to resolve this, you can add the following config to your tach.toml:

[external]
rename = ["PIL:pillow"]

In general, the rename field takes a list of strings in the format [module name]:[package name].