Remove subprocess usage, remove spurious direnv calls

[gforsyth]

This removes the usage of subprocess directly in favor of using xonsh to make the initial direnv call.

It also removes the precommand hook, since the combination of chdir and postcommand should cover just about everything (especially changes to watched files).

Finally, the chdir event only fires when we change into a new direnv dir, and not every time we move down the directory tree, so we aren't constantly reloading the same envrc (which can get quite slow when using nix)

[Mic92]

Another advantage is also that this variant can be cancled with ctrl-c. Before it would close the whole shell, now it kills direnv.

[Mic92]

Works for me!

[74th]

Thanks for the pull request.
The suggestion to remove subprocess is a good one.

The change to chdir, won't .envrc in a subdirectory stop working if the subdirectory also has .envrc?

But now that I run xonsh-direnv, I see that on_postcommand works even with "cd subdir".

[gforsyth]

@74th -- yep, I think it should still work.

[74th]

This works fine! Thank you!