Allow overriding individual exports of reexported external namespaces

[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 #3137

Description

This changes how external namespaces are reexported by moving the relevant code to the end and skipping local exports. In a previous PR, it was ensured that local exports are always defined as properties on the exports object at this point even if they have not yet been assigned a value.

[rollup-bot]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#override-namespace-export

or load it into the REPL:
https://rollupjs.org/repl/?circleci=14300

[codecov]

Codecov Report

Merging #3959 (272ffd9) into master (0e3445b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3959   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files         191      191           
  Lines        6703     6703           
  Branches     1960     1960           
=======================================
  Hits         6515     6515           
  Misses         99       99           
  Partials       89       89           

Impacted Files	Coverage Δ
src/finalisers/shared/getExportBlock.ts	100.00% <100.00%> (ø)

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