fix(inject): Inject regardless of whether a sourcemap is present

[loewenheim]

This removes the checks on whether a source file references a sourcemap and whether that sourcemap can be found in the local file system. Instead, it now

1. always injects minified source files (unless they already have a debug id, of course)
2. injects the same debug id into the sourcemap file, if applicable
3. reuses the debug id in the sourcemap file, if any.

[iambriccardo]

LGTM