Download translator/onboarding packages (and separate dependency packages) from an NPM registry to a local cache

[tjaffri]

#2 contains the API stubs only for this.

This may use a fork of NPM that we modify to be compatible with embedded Node.js, or maybe some other solution. Requires more investigation.

There will be two distinct services involved:

1. An NPM registry, that tells you exactly where you can download packages and dependencies, when you already know what packages you need. Initially we can just use the public NPM registry, but eventually we'll want to use a Microsoft-hosted mirror. The client API for this is covered by this task.
2. What I'm calling the "resolve" service, that tells you what onboarding & translator packages you need to get, given some metadata about a thing. Client APIs for this service are covered in a separate task later.

[jasongin]

APIs for the "resolve" service mentioned above are tracked as #9

[jasongin]

Unimplemented API stubs for this are in #2 as the NpmPackageSource class.