Fix hasShadowedGlobals check to properly ignore top-level declarations

[alangpierce]

We'll want top-level declaration detection for #228 anyway, and
hasShadowedGlobals was buggy because it was treating export declarations as
shadowing themselves, so this adds some bookkeeping so we know when a
declaration is top-level or not.

This doesn't affect correctness, it was just an optimization that wasn't kicking
in when it was supposed to.

Tracking scope depth throughout the parser is a little scary and adds another
field to the state, so there may be a way to improve it in the future, but it
certainly should work for now.

[codecov-io]

Codecov Report

Merging #367 into master will increase coverage by 0.12%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master     #367      +/-   ##
==========================================
+ Coverage   79.47%   79.59%   +0.12%     
==========================================
  Files          44       44              
  Lines        5066     5101      +35     
  Branches     1131     1133       +2     
==========================================
+ Hits         4026     4060      +34     
- Misses        741      742       +1     
  Partials      299      299

Impacted Files	Coverage Δ
src/identifyShadowedGlobals.ts	87.8% <ø> (+0.3%)	⬆️
src/parser/traverser/expression.ts	82.92% <100%> (+0.45%)	⬆️
src/transformers/CJSImportTransformer.ts	92.02% <100%> (-0.03%)	⬇️
src/parser/tokenizer/state.ts	100% <100%> (ø)	⬆️
src/parser/plugins/flow.ts	62.21% <100%> (+0.07%)	⬆️
src/parser/traverser/lval.ts	97.5% <100%> (+0.06%)	⬆️
src/parser/tokenizer/index.ts	72.68% <100%> (+0.59%)	⬆️
src/parser/traverser/statement.ts	80.28% <88.23%> (+0.17%)	⬆️

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