Make sure this.load waits for modules that are already loading

[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 #4294

Description

this.load is meant to wait until the code of a module is available. This was not the case if a module was already loading for some other reason.

[github-actions]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#gh-4294-preloaded-code

or load it into the REPL:
https://rollupjs.org/repl/?pr=4296

[codecov]

Codecov Report

Merging #4296 (777f7c8) into master (e6d70d8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4296   +/-   ##
=======================================
  Coverage   98.45%   98.45%           
=======================================
  Files         205      205           
  Lines        7311     7311           
  Branches     2082     2082           
=======================================
  Hits         7198     7198           
  Misses         55       55           
  Partials       58       58           

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

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