Make sure uninitialised exports turn up via .hasOwnProperty for non-ES formats

[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

This will make sure that even uninitialised named exports will create properties on the exports object in CJS/IIFE/AMD/UMD formats so that checking for those properties via .hasOwnProperty or Object.keys works.
This is important for tools that rely on this for e.g. merging and forwarding namespaces.

This was already working properly for constant exports but was broken for exports that were assigned dynamically later, example:

https://rollupjs.org/repl/?version=2.38.5&shareable=JTdCJTIybW9kdWxlcyUyMiUzQSU1QiU3QiUyMm5hbWUlMjIlM0ElMjJtYWluLmpzJTIyJTJDJTIyY29kZSUyMiUzQSUyMmV4cG9ydCUyMGxldCUyMGZvbyUzQiU1Q24lNUNuZXhwb3J0JTIwY29uc3QlMjB1cGRhdGVGb28lMjAlM0QlMjAoKSUyMCUzRCUzRSUyMGZvbyUyMCUzRCUyMCd1cGRhdGVkJyUyMiUyQyUyMmlzRW50cnklMjIlM0F0cnVlJTdEJTVEJTJDJTIyb3B0aW9ucyUyMiUzQSU3QiUyMmZvcm1hdCUyMiUzQSUyMmNqcyUyMiUyQyUyMm5hbWUlMjIlM0ElMjJteUJ1bmRsZSUyMiUyQyUyMmFtZCUyMiUzQSU3QiUyMmlkJTIyJTNBJTIyJTIyJTdEJTJDJTIyZ2xvYmFscyUyMiUzQSU3QiU3RCU3RCUyQyUyMmV4YW1wbGUlMjIlM0FudWxsJTdE

Note that when updateFoo is not run, the property foo will no exist.

[rollup-bot]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#uninitialised-exports

or load it into the REPL:
https://rollupjs.org/repl/?circleci=14279

[codecov]

Codecov Report

Merging #3957 (e7a3f36) into master (45d8e5e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3957   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files         190      191    +1     
  Lines        6694     6694           
  Branches     1958     1958           
=======================================
  Hits         6506     6506           
  Misses         99       99           
  Partials       89       89           

Impacted Files	Coverage Δ
src/ast/nodes/VariableDeclaration.ts	97.87% <100.00%> (-0.13%)	⬇️
src/ast/nodes/VariableDeclarator.ts	100.00% <100.00%> (ø)
src/utils/reassignedExportsMember.ts	100.00% <100.00%> (ø)

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