FIX: when processing sourcemap files that do not exist, also remove nested transformation extensions

[markrmullan]

The requirements about file extensions for sourcemaps, per the spec (https://sourcemaps.info/spec.html#h.9ppdoan5f016), are hazy at best. There is a recommendation that sourcemap file names may match the generated file but with a .map extension, but it appears they do not explicitly need to end with .map.

On a similar note, sourcemap files MUST contain one valid JSON object (https://sourcemaps.info/spec.html#h.1ce2c87bpj24) and as a result, .map.json could also represent a valid file extension. Browsers also attempt to fetch .map.json sourcemaps if they are specified as a sourceMappingURL, hinting that they are indeed valid:

Previously, the removed_sourcemap_comment method in the SourcemappingUrlProcessor would strip the sourceMappingURL=file.js.map comment from the digested asset if the sourcemap file did not exist, but it would not remove nested file extension transformations. This caused an issue for us because the end result would be an uncommented piece of .json code, which of course broke the script:

This PR ensures that if the sourceMappingURL comment references a sourcemap file with a nested file extension that does not exist, that the nested file extension itself is also removed when the //# sourceMappingURL comment is scrubbed.

[markrmullan]

Hey @rafaelfranca 👋 just wanted to bump this PR and see if you had any thoughts / concerns. Thank you + other contributors for your blood, sweat and tears on this project and much more!