OpamSystem.t_resolve_command: use ocaml code for looking up commands in PATH

[Armael]

...instead of calling the shell (on linux).

The motivation is performance. It turns out that the speedup of #4064 is due not only to less time being spent launching processes, but also less time being spent calling to the shell in order to resolve the command. Doing the resolution (using PATH) directly in ocaml should be faster.

Note that command resolution was already done in ocaml on Windows, so this PR mostly merges the (previously) windows-specific code with the linux code.

The PR also attempts to streamline a bit the handling of Not_found vs Denied. The behavior is mostly unchanged when looking up a fully specified path (I added a Not_found case for increased precision; previously, a non-existing binary would be reported as "denied").
When a command is looked up in PATH, I follow the shell behavior a bit more consistently (as specified here "Other environment variables) and skip binaries that do not have the right permissions.
If no suitable binary is found, then Not_found is returned (the previous code was inconsistent: it would've returned Denied under windows, but Not_found under linux).

[Armael]

(incidentally, this PR happens to be fixing the cygwin CI (!))

[AltGr]

This is great, thanks a bunch :)

[dra27]

Coming very late to this, for which apologies - this didn't fix Cygwin CI, it masked the problem, owing I think to a bug in the implementation. Beforehand, command -v cmd was correctly returning /cygdrive/c/WINDOWS/system32/cmd and the permissions check was failing, so an exception was raised. After this change, the function considers /cygdrive/c/WINDOWS/system32 in PATH but because it fails the permissions check (as it was before) it continues on to the next item in PATH. Except it never finds cmd in any other directory so now resolving the command returns `Not_found instead of `Denied.

Failing the permissions check is valid to skip on to the next directory in PATH, but it's incorrect (I think) to return `Not_found if any of the directories did contain the command, just with the appropriate permissions.

[Armael]

Ah, wow, that makes sense indeed. Sorry for introducing that trick bug!