tools/direnv: replace direnv package with envrc

wbolster/emacs-direnv uses an approach that mutates global state,
allowing a direnv to bleed into unrelated buffers and contexts. For this
to work it must rereads the direnv every time you switch buffers, which
can be very slow.

purcell/envrc, on the other hand, makes env state buffer-local, so the
direnv only needs to be read once, when a is first initialized. This is
faster and less error prone. However, it's necessary to manually reload
the direnv if you've changed your .envrc outside of Emacs (with `M-x
envrc-reload` or `M-x envrc-reload-all`).

modules/tools/direnv/README.org

@@ -34,7 +34,7 @@ direnv and then made available to the current shell.
 This module provides no flags.
 
 ** Plugins
-+ [[https://github.com/wbolster/emacs-direnv][direnv]]
++ [[https://github.com/purcell/envrc][envrc]]
 
 ** Hacks
 + Normally, the direnv environment is updated on ~post-command-hook~. We've