install-files shouldn't run if its client is being installed directly

[wearhere]

Install scripts, thus install-files, run both when the package using install-files is installed directly as well as when it's installed as a dependency of another package. We only want install-files to run in the latter scenario since it's supposed to copy the files to that dependent.

To use the example from the README, install-files should run if the user runs npm install my-ebextensions from my-microservice's directory. It should not run if the user cds into my-ebextensions and runs npm install there.

[wearhere]

This is a minor bug since install-files packages don't run standalone so you basically never need to run npm install inside them, at least I don't. You could work around this by adding the install script last when developing such a package, or avoiding even doing npm install install-files but rather just copying the package.json from another install-files package.