This change implements the Node.js language host's GetRequiredPlugins
function. This merely scans all node_modules/*/package.json files in
the program directory, looking for those that have associated plugins.
It returns a list of any found along with their version numbers.
This addresses pulumi/pulumi#446: what we used to call "package" is
now called "project". This has gotten more confusing over time, now
that we're doing real package management.
Also fixespulumi/pulumi#426, while in here.
