[infra] fix vendor import paths on built files

[infra] fix vendor import paths on built files

A previous commit started (quasi-correctly) rewriting vendor import
paths on built files.  The gist of it was correct, but it incorrectly
rewrote paths in vendor itself.

The babel plugin in use operated on canonical absolute paths.  This mean
that it saw no difference between the import
`../vendor/pako/lib/utils/foo` and `../utils/foo`, where the later was
actually in the `vendor/pako/lib/bar` directory.  This rewrote imports
in files in the vendor directory itself.  However, since those files
were *already* in the correct relative location, the new import was
incorrect by a degree of `..`.

Now, we only rewrite vendor paths on things in the `core` directory.