Set entrypoints to use ./lib rather than ./src (#12906)

Currently, we replace the entrypoints in package.json during the release
cycle. I think, historically, this was done to make element-web development
easier, but that doesn't actually use these entrypoints (instead it imports
from `src`).

Accordingly, I think the switcheroo is unnecessary; furthermore it causes a
whole bunch of confusion by making the development environment different from
the release environment, and it complicates our CI and release process.

In short, the switcheroo has to die.

(cherry picked from commit 2768b9cb)