Ensure system-reserved identifiers are deduped

[guybedford]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

Fixes #3613

[rollup-bot]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#system-reserved

or load it into the REPL:
https://rollupjs.org/repl/?circleci=11635

[codecov]

Codecov Report

Merging #3614 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3614   +/-   ##
=======================================
  Coverage   96.39%   96.39%           
=======================================
  Files         182      182           
  Lines        6210     6212    +2     
  Branches     1820     1819    -1     
=======================================
+ Hits         5986     5988    +2     
  Misses        111      111           
  Partials      113      113           

Impacted Files	Coverage Δ
src/Chunk.ts	99.79% <100.00%> (+<0.01%)	⬆️

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[lukastaegert]

Thanks for the quick response 👍