Running multiple different npm install operations doesn't work

[Turbo87]

As mentioned in a1a5f6f it is currently unreliable to run multiple NPM operations in the same process, even when run in sequence. The reason for that is NPM setting a loaded flag and "loading" the NPM code a second time with different options (e.g. with and without --save-dev) will not activate the new options.

NPM has also stated in the past (reference missing unfortunately) that using their code programmatically is not supported and people should use exec and run the CLI tool instead.

#6461 has already started work in a similar direction, but might take longer to get merged since it's changing a few other things as well.