Convert import script to a go test

[JonathanOppenheimer]

Why this should be merged

This closes #813:

The shell script ./scripts/lint_allowed_eth_imports.sh is getting rather complex and hard to understand, we should migrate it to use Go as a test, especially now that we have getDependencies code which can easily be adapted for this use case.

I was unable to adapt the getDependencies() function to work properly with ignoring _ imports and imports that start with eth, but I did massively improve the test, and this now catches some imports that the shell script did not.

I will remove getDependencies() in the next PR anyway.

Sample:

How this works

How this was tested

The new TestLibevmImportsAreAllowed test

Need to be documented?

No

Need to update RELEASES.md?

No

[JonathanOppenheimer]

This is blocked by #1349 and #1350