[ExternalSource] Store downloaded external source podspecs as JSON, evaluated from the context of the source directory

Closes https://github.com/CocoaPods/CocoaPods/issues/2875.
Depends upon https://github.com/CocoaPods/Core/pull/203.
Also stores podspecs in the sandbox as JSON.
I will write up a changelog entry if this approach is deemed acceptable.

\c @alloy @kylef