Fix mangling of Named Exports in Modules

[boopathi]

• Ref Mangling exports #479

This completes one part of the problem where now Named exports are no
more mangled. But the other part, where we can convert

const foo = bar;
export {foo};

to

export const foo = bar;

is yet to be explored in DCE or other plugins.

[codecov]

Codecov Report

Merging #518 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #518      +/-   ##
==========================================
+ Coverage   82.96%   83.11%   +0.14%     
==========================================
  Files          34       34              
  Lines        2524     2570      +46     
  Branches      897      922      +25     
==========================================
+ Hits         2094     2136      +42     
- Misses        257      258       +1     
- Partials      173      176       +3

Impacted Files	Coverage Δ
...ages/babel-plugin-minify-mangle-names/src/index.js	77.64% <100%> (+1.1%)	⬆️
packages/babel-plugin-minify-simplify/src/index.js	80.9% <0%> (+0.4%)	⬆️

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