Handle string type references in cast()

[Kronuz]

Summary

Handle cast("Type", object) so Type is properly referenced in the scope.

Test Plan

Run tests

[codecov-io]

Codecov Report

Merging #418 (e30a8c4) into master (7478d73) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #418   +/-   ##
=======================================
  Coverage   94.30%   94.30%           
=======================================
  Files         232      232           
  Lines       22437    22460   +23     
=======================================
+ Hits        21159    21182   +23     
  Misses       1278     1278           

Impacted Files	Coverage Δ
libcst/metadata/scope_provider.py	96.73% <100.00%> (+0.03%)	⬆️
libcst/metadata/tests/test_scope_provider.py	100.00% <100.00%> (ø)

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