fix: breakpoints in extension subdirs on windows not working (microsoft#184195)

It did not normalize to forward slashes, so the empty sourcemap was
created with a source called `foo\\bar.js`. Then the build process was
adding a new source `foo/bar.js` since it didn't see it already in the
sourcemap, but `foo\\bar.js` was later normalized. This normalized
version came first in the mapping, but didn't have any mapped locations,
so breakpoints never got set.

Author: connor4312