Add warning for implicit relative imports issues

[Eddy114514]

Python 2 supports implicit relative imports inside packages. A bare import inside a package can resolve to a sibling module.

Example package:

pkg/
  __init__.py
  foo.py
  main.py

# pkg/main.py
import foo

In Python 2, this can resolve to:

pkg.foo

In Python 3, imports are absolute by default, so the same code resolves as:

foo

This means the code can import a different top-level module or fail with ImportError.

For same environment of

# pkg/main.py
import foo

Python 2 resolves to pkg.foo
Python 3 resolves to top-level foo

I modified import.c to fire warning only if

1. The import uses Python 2 implicit-relative semantics.
2. The importing module has package context.
3. The import is bare, not explicit-relative.
4. Runtime resolution actually selected a package sibling module.

To ensure above, I checks whether the resolved module name is:
current_package + "." + imported_name
For example:

pkg + "." + foo == pkg.foo
If that condition is true, the import depended on Python 2 implicit relative import behavior, so pygrate2 emits a warning.