With webpack 5 and source-map-loader, source map parsing fails

[fwextensions]

Describe the bug
When using source-map-loader in webpack 5 to include source maps from node modules, I'm getting this warning when building my code:

WARNING in ./node_modules/@watergis/mapbox-gl-legend/dist/index.js
Module Warning (from ./node_modules/source-map-loader/dist/cjs.js):
Failed to parse source map from 'C:\ ... \node_modules\@watergis\mapbox-gl-legend\lib\index.ts' file:
Error: ENOENT: no such file or directory, open 'C:\ ... \node_modules\\@watergis\mapbox-gl-legend\lib\index.ts'

I'm not sure why it's looking for index.ts when loading the source maps, but I think I didn't see this with webpack 4, and haven't seen issues with other modules.

To Reproduce
The webpack.config.js file for this project includes this rule:

    module: {
      rules: [
       {
          test: /\.js$/,
          enforce: 'pre',
          use: ['source-map-loader'],
        }

It's including source-map-loader v3.0.0.

Expected behavior
The source map should be included in dev builds, as it is with other npm modules.

Desktop (please complete the following information):

• OS: Windows 10
• Browser: Chrome
• Version: 95

[fwextensions]

With v1.2.1 of the legend module, I also see this warning:

WARNING in ./node_modules/@watergis/mapbox-gl-legend/dist/legend-control.js
Module Warning (from ./node_modules/source-map-loader/dist/cjs.js):
Failed to parse source map from 'C:\ ... \node_modules@watergis\mapbox-gl-legend\lib\legend-control.
ts' file: Error: ENOENT: no such file or directory, open 'C:\ ... \node_modules@watergis\mapbox-gl-legend\lib\legend-control.ts'