Prioritize manual chunk name over dynamic entry id

[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 #3973

Description

If a manual chunk only contains a single dynamic entry point, the manual chunk name would be overridden by the name derived from the dynamic entry. As this is not in the user's interest, this will change the priorities to prefer the user-given name.
Now the only situation where a manual chunk will not receive the expected name is if there is a single explicitly defined entry point in the chunk (which is an optimisation to reduce the number of chunks).

[github-actions]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#gh-3973_better_manual_chunk_names

or load it into the REPL:
https://rollupjs.org/repl/?circleci=14949

[codecov]

Codecov Report

Merging #4040 (d078ef9) into master (345ae5d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4040   +/-   ##
=======================================
  Coverage   97.47%   97.48%           
=======================================
  Files         192      192           
  Lines        6785     6788    +3     
  Branches     1994     1996    +2     
=======================================
+ Hits         6614     6617    +3     
  Misses         84       84           
  Partials       87       87           

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

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