API for acquiring types

[bowdenk7]

We should provide a way for others to ask for types that Salsa will fetch.

We use bower.json and package.json as sources for fetching types, but Cordova projects have an additional source config.xml which lists plugins that also need types. We don't want to check config.xml for all projects, nor be dependant on the structure of config.xml going forward. But we should allow Cordova to parse it themselves and send us a list of libs that they want types for.

[mhegazy]

This looks like an extensibility point for discoverTypings. Related to #6508

[mhegazy]

//cc:@weswigham