Warnings in checkLanguageKeys when a source map refers to non-bundled files

[mwoc]

Running checkLanguageKeys in a project using bootstrap.css (which has a source map), and assetgraph-builder v4.10.9.

One of the transforms finds the source map, and in that finds references to sources which are not bundled, throwing WARN messages:

⚠ WARN: http-pub/3rdparty/bootstrap/css/less/normalize.less: ENOENT: no such file or directory, open 'http-pub/3rdparty/bootstrap/css/less/normalize.less'
Including assets:
http-pub/3rdparty/bootstrap/css/bootstrap.css.map

I'm unsure what the behaviour regarding source maps should be in checkLanguageKeys though :)