Do not warn unnecessarily for namespace conflicts

[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:

Description

Previously, the logic to resolve exports especially across conflicting namespace reexports was separate from the logic that would warn about conflicting namespace reexports.

In fact, the latter logic was broken in so far as it warned about situations like

// main.js
export const foo = 'main';
export * from './other.js';

// other.js
export const foo = 'other';

There is no reason to warn here as explicit (re)exports always take precedence over namespace reexports.

This refactors the entire logic and merges both mechanisms. This has the advantage that you only get a warning when a conflicting export is actually used, which is in line with how it works in actual JS runtimes.

[github-actions]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#namespace-conflicts

or load it into the REPL:
https://rollupjs.org/repl/?pr=4363

[codecov]

Codecov Report

Merging #4363 (27780bf) into master (3ca594e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4363      +/-   ##
==========================================
- Coverage   98.69%   98.68%   -0.01%     
==========================================
  Files         205      205              
  Lines        7339     7326      -13     
  Branches     2089     2083       -6     
==========================================
- Hits         7243     7230      -13     
  Misses         36       36              
  Partials       60       60              

Impacted Files	Coverage Δ
src/Graph.ts	100.00% <ø> (ø)
src/ExternalModule.ts	100.00% <100.00%> (ø)
src/Module.ts	100.00% <100.00%> (ø)
src/utils/error.ts	100.00% <100.00%> (ø)

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