In gh-4466, we removed the `external` directory in favor of loading some files
directly from `node_modules`. This works fine locally but when deploying code
for tests, this makes it impossible to not deploy `node_modules` as well. To
avoid the issue, this change restores usage of the `external` directory.
One change is that we no longer commit this directory to the repository, its
only purpose is to have clear isolation from `node_modules`.
Ref gh-4466
Closess gh-4865
Now that Sizzle is gone & we use npm, we can read from node_modules directly
and skip the setup that copies some files to the external directory.
Closes gh-4466
