Pin accesses to import alias node

[giomeg]

Summary

Add ImportAssignment class extending Assignment to record assignments for import statements.

Test Plan

python -m unittest libcst.metadata.tests.test_scope_provider
python -m unittest libcst.codemod.visitors.tests.test_gather_unused_imports

Closes #485

[codecov-commenter]

Codecov Report

Merging #554 (21d1471) into main (5c05001) will decrease coverage by 0.00%.
The diff coverage is 97.05%.

@@            Coverage Diff             @@
##             main     #554      +/-   ##
==========================================
- Coverage   94.77%   94.77%   -0.01%     
==========================================
  Files         235      235              
  Lines       23254    23273      +19     
==========================================
+ Hits        22040    22058      +18     
- Misses       1214     1215       +1     

Impacted Files	Coverage Δ
libcst/codemod/visitors/_gather_unused_imports.py	98.33% <ø> (ø)
libcst/metadata/__init__.py	100.00% <ø> (ø)
libcst/metadata/scope_provider.py	95.64% <95.65%> (+0.07%)	⬆️
libcst/metadata/tests/test_scope_provider.py	99.88% <100.00%> (+<0.01%)	⬆️
setup.py	87.50% <0.00%> (-12.50%)	⬇️

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[lpetre]

This looks great to me, thanks for working on this.

[zsol]

lgtm