Add sourcemapBaseUrl option

[nickgarlis]

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 feature can be useful for when sourcemaps need to be loaded through a different URL to the one where the bundled code is loaded from.

It would be the equivalent of the publicPath option in Webpack's source-map-dev-tool plugin

The use case would be to source my organisation's sourcemaps in a server within our internal network to prevent exposing the source code on production.

If this is something of interest then the following features could also be considered:

• Providing a base URL option for bundle sources (sources field inside of *.map files)
• Providing a different output directory for sourcemaps.

[codecov]

Codecov Report

Merging #4527 (b922bc3) into master (fbb86f5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4527   +/-   ##
=======================================
  Coverage   98.86%   98.87%           
=======================================
  Files         208      209    +1     
  Lines        7334     7346   +12     
  Branches     2095     2098    +3     
=======================================
+ Hits         7251     7263   +12     
  Misses         27       27           
  Partials       56       56           

Impacted Files	Coverage Δ
src/utils/options/mergeOptions.ts	100.00% <ø> (ø)
src/rollup/rollup.ts	100.00% <100.00%> (ø)
src/utils/options/normalizeOutputOptions.ts	100.00% <100.00%> (ø)
src/utils/url.ts	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fbb86f5...b922bc3. Read the comment docs.

[nickgarlis]

I am not sure where I should add tests for functions like these. It looks like all of the tests in the project are end-to-end tests and do not import typescript code from src. @lukastaegert I'd be happy to open a separate PR adding unit tests that execute with mocha -r ts-node/register.

[lukastaegert]

No, please don't. Then we would still need a test that actually uses the option, i.e. an E2E test. I mean that is the point, to test the effect of this option. As e2e tests in Rollup are REALLY fast, there is really no point in writing unit tests that need to be adapted with every refactoring.
A very simple way could be to add a chunking-form test because those tests just snapshot the output directory and that would include the source map. Or you add a sourcemap test, those tests pass you the source map and you make assertions on the generated map. If all else fails, I will need to look into writing a test myself.

[lukastaegert]

I took the liberty to add a test for the bad case, which also prompted me to improve the error message a little. I also added some documentation for the option itself and shortened the CLI printout to match the 80 character line-break we use in that file. Otherwise, this is really high quality work and I plan to merge it once everything is green.