Maybe convert xsh to py for faster startup?

[eugenesvk]

Since xonsh xontribs are not compiled unlike the .py ones, so take slower to start xonsh/xonsh#3953, it might make sense to rewrite this xontrib in Python?

I've taken a stab at it by changing the pyenv version (and the langenv_common) in this fork https://github.com/eugenesvk/xontrib-langenv/tree/repy, besides conversion I've also didn't understand why there is a need to search for tools themselves (not their shims) that are supposed to be in PATH already if they're properly installed, so just replaced it ;) with a lazy search of known xonsh binaries

upd: seems to be working, also #14 is fixed
However, I don't get how source-bash works, it doesn't print anything even in the regular subprocess, e.g. source-bash --suppress-skip-message -n "echo 1" doesn't print 1, so in the create_alias function when you do pyenv shell which ultimately goes to source-bash to source echo $PYTHON_VERSION, nothing is printed even if said variable exists

asked this question in xonsh discussions xonsh/xonsh#5019