Fix finding psa on Windows (#693)

[stkb]

In RunEnv.hs:getPurs we were just using the standard Directory.findExecutable to look for psa, forgetting to check for psa.cmd too. In windows this was looking for psa.exe, which doesn't exist.

To help prevent this happening again, a self-written findExecutable function in Prelude is added, instead of just exporting
Directory.findExecutable. This new version always first checks for a .cmd version of the given executable name on Windows.

Fix #693

[stkb]

Waiting for CI tests. It's working for me in standard Windows/Cmd.

> spago -v build --purs-args "--stash"
[debug] Running `getGlobalCacheDir`
[debug] Transformed config is the same as the read one, not overwriting it
[debug] Ensuring that the package set is frozen
[debug] Running `spago build`
[debug] Getting transitive deps
[debug] Running `fetchPackages`
[debug] Checking if `purs` is up to date
[debug] Compiling with "psa"
[debug] Running command: `psa compile --stash ".spago/console/v4.4.0/src/**/*.purs" ".spago/effect/v2.0.1/src/**/*.purs" ".spago/prelude/v4.1.1/src/**/*.purs" ".spago/psci-support/v4.0.0/src/**/*.purs" "src/**/*.purs" "test/**/*.purs"`
           Src   Lib   All
Warnings   0     0     0
Errors     0     0     0
[info] Build succeeded.

[f-f]

Thank you @stkb!!

I left a small comment, but otherwise I think we're good to merge 🚀

Also CI seems to be failing because of some leftover import (we have --pedantic set when building PRs on GitHub Actions)

[f-f]

@ntwilson would you be able to verify that this works for you? (to try out this version of Spago it should be enough to checkout the branch and running stack install)

[ntwilson]

@ntwilson would you be able to verify that this works for you? (to try out this version of Spago it should be enough to checkout the branch and running stack install)

Yes, it worked!! Thanks, @stkb!
I only tested the global version. Do we have any reason to expect it would work differently through npm/npx? Is there a way to test that from a local build?

[stkb]

@f-f Oops yeah missed that 🤦‍♂️. Also I wasn't seeing the redundant import error on my machine because I was still using an old Stack version. Anyway it's all looking good now 👍

@ntwilson Good to hear and I don't see any reason why it shouldn't work if installed through npm.

[f-f]

Thank you both! 🙂