Add Source Map sourceRoot Support

[jridgewell]

This adds the --source-root, sourceRoot (JS), and SourceRoot (Go) options to insert a sourceRoot field in any output source maps. When a sourceRoot exists on the map, any sources are resolved relative to it.

This is particularly useful when hosting compiled code on a server, and pointing the source files to a GitHub repo. Eg, as AMP does.

Fixes #675.

[evanw]

Some thoughts:

• I think this should be sourceRoot instead of sourcemapRoot. This matches the name of the property in the source map, and esbuild's sourcesContent setting is an existing precedent for these settings matching the name of the corresponding source map property.

• Right now the paths in the source map are relative to the absWorkingDir setting. Another option could be to change this to be relative to the outbase setting instead. I'm wondering which one is more appropriate.

[jridgewell]

I think this should be sourceRoot instead of sourcemapRoot.

Sure, I can make that change tonight.

Right now the paths in the source map are relative to the absWorkingDir setting.

I actually like that it's relative to absWorkingDir, although setting it relative to could work. My experience is with Closure compiler, which uses the working directory. Rollup seems to make everything relative to the directory of the output sourcemap (thought they provide a function option to transform it). Webpack seems to use working directory, though they use an absolute webpack://package/PATH URL.

When making npm packages, I generally don't include the src directory in the package's published files. So making the sources relative to the build directory isn't correct, I use the sourceRoot to point to the git code at the specific version tag.

Perhaps we could just create an option for sourcemapOutbase, and have it default to whichever of absWorkingDir or outbase you prefer?

[jridgewell]

This has been updated to --source-root, sourceRoot (JS), and SourceRoot (Go).

[evanw]

Looks reasonable to me! Thanks for the PR.