Avoid variable name conflict when reexporting several namespaces from a chunk

[lukastaegert]

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:
Resolves #3741

Description

The problem was that an optimization meant for preserveModules was also applied in all other cases where it did not make sense and was indeed harmful.

[rollup-bot]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#gh-3741-namespace-name-conflict

or load it into the REPL:
https://rollupjs.org/repl/?circleci=12697

[codecov]

Codecov Report

Merging #3742 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3742   +/-   ##
=======================================
  Coverage   96.99%   96.99%           
=======================================
  Files         184      184           
  Lines        6452     6453    +1     
  Branches     1872     1873    +1     
=======================================
+ Hits         6258     6259    +1     
  Misses        103      103           
  Partials       91       91           

Impacted Files	Coverage Δ
src/utils/deconflictChunk.ts	100.00% <100.00%> (ø)

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