Change how npm.cmd is located in NpmHelper [hotfix_fake4]

[nikolaia]

I've noticed that Yarn adds nodejs\bin to the path variable, which in the old code could return a folder that did not contain the npm.cmd file. I fixed this and also removed the fallback to the npm.js package, as it is silly to assume where the package directory is. If people are using that package they should overwrite the NpmFilePath variable.

This is the same as #1629 just against the hotfix_fake4 branch

[matthid]

Is this a hotfix?

Note that the fallback to "./packages/Npm.js/tools/npm.cmd" assumes that one uses the https://www.nuget.org/packages/Npm.js package in combination with Paket.
It looks a bit outdated?

[nikolaia]

The old way was a bit naive and broke in a lot of situation, the most noticeable one being that it would fail if you have a value containing nodejs in your PATH env variable that didn't contain npm.cmd. An example of that could be C:\Program FIles\nodejs\bin - while npm.cmd is located in C:\Program FIles\nodejs that comes later in your PATH env var.

The fallback to the package, as you say, assumes that you actuallyt use the package together with Paket. The package seems outdated and it just makes for an unclear error message.

[nikolaia]

Did you not consider this to be a hotfix for FAKE4? The dependency on/fallback to the outdated Npm.js nuget package can probably be considered broken, and this PR changes it to look more thorough trough the PATH variable for npm.exe. Basically same behaviour as on *nix where it assumes you install NPM yourself.

[nikolaia]

Solved for FAKE5 in #1822

[matthid]

Did you not consider this to be a hotfix for FAKE4?

Problem is that I cannot release fake 4 for some reason. Also at this time I don't think there is a good reason to stay on fake 4 anymore. The first step of the migration to fake 5 (upgrading the nuget package to v5) should be non-breaking.