fix[lang]: fix exception for duplicate builtin imports

[Sporarum]

What I did

Fix #4603

How I did it

Add new dict which keeps track on built-ins
It might be possible to do it with self.graph, but I don't know enough about it yet

How to verify it

Commit message

this commits refactors the way imports are checked to provide a more
user-friendly error message to users. this is done by moving the custom
handling of builtin functions after the duplicate check.

Description for the changelog

Fix error message when built-in modules are imported more than once

Cute Animal Picture

[Sporarum]

Fixes #4603

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.07%. Comparing base (79d0762) to head (985f10c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4764   +/-   ##
=======================================
  Coverage   93.07%   93.07%           
=======================================
  Files         145      145           
  Lines       20366    20366           
  Branches     3578     3578           
=======================================
  Hits        18956    18956           
  Misses        949      949           
  Partials      461      461           

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[charles-cooper]

this compiles on master but not on this branch:

from ethereum.ercs import IERC20 as builtin_IERC20      # builtin             
from .ethereum.ercs import IERC20 as local_IERC20       # local file