Disable treeshaking per module

[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 #3655, resolves #3651

Description

This allows to disable tree-shaking on a per-module basis by setting moduleSideEffects to "no-treeshake" in https://rollupjs.org/guide/en/#resolveid, https://rollupjs.org/guide/en/#load or https://rollupjs.org/guide/en/#transform.

Besides the obvious effect, this will also make sure that this module is always included in one of the generated chunks, even if the module is empty. The important use-case here is CSS (or in general asset) import handling when the imports should later be associated with a specific chunk:

1. The plugin that handles e.g. import 'foo.css' returns {code: '', moduleSideEffects: 'no-treeshake'} in its load hook for "foo.css".
2. Then in generateBundle, the plugin can scan the modules properties in the chunks of the passed bundle object for our module foo.css and associate the actually generated CSS correctly with the given chunk.

[rollup-bot]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#disable-module-treeshaking

or load it into the REPL:
https://rollupjs.org/repl/?circleci=12276

[codecov]

Codecov Report

Merging #3663 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3663   +/-   ##
=======================================
  Coverage   96.71%   96.71%           
=======================================
  Files         183      183           
  Lines        6299     6303    +4     
  Branches     1834     1835    +1     
=======================================
+ Hits         6092     6096    +4     
  Misses        105      105           
  Partials      102      102           

Impacted Files	Coverage Δ
src/ExternalModule.ts	97.95% <ø> (ø)
src/Graph.ts	100.00% <100.00%> (ø)
src/Module.ts	100.00% <100.00%> (ø)
src/ModuleLoader.ts	100.00% <100.00%> (ø)
src/utils/transform.ts	98.43% <100.00%> (ø)

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