There is a reoccuring problem in readTree-type repositories that use
nix-shell, where evaluation of the full set of dependencies that
should be made available to users takes a noticeable amount of time,
slowing down operations when `direnv` is involved.
In depot, we have so far fixed this by maintaining a manual `//bin`
directory which contains a set of symlinks to a central dispatch
script that can dispatch to various tools in depot lazily.
This script can instead be generated ad-hoc by Nix (pretty fast if we
can make assumptions like `git` and `nix-build` existing on user's
machines already) and added to $PATH.
The function introduced in this commit implements the logic for that.
The structure of the script is based on the existing
`//bin/__dispatch`.
This does not yet switch depot's envrc to use this new method of
installing dependencies lazily.
Change-Id: I92efcd9bb6aa51aa2709ad910a464e9dac97ee89
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5512
Tested-by: BuildkiteCI
Reviewed-by: ezemtsov <eugene.zemtsov@gmail.com>
