Implement Lock file functionality

[vincentparrett]

A lock file will enable repeatable/reliable restores across multiple machines, regardless of package sources. In particular, the is required to guarantee that CI builds are using the same versions of transitive dependencies that the developers are using.

See

https://github.com/NuGet/Home/wiki/Enable-repeatable-package-restore-using-lock-file https://fsprojects.github.io/Paket/lock-file.html

for ideas.

This will require changes to the resolution code.

[vincentparrett]

Implemented - transient references are recorded under the direct dependencies. No need for a lock file as this achieves the same result.