Make `migrateToBazel` and dependant tasks cacheable.

[arunkumar9t2]

Currently explicitly caching is disabled due to complexity but we need to consider adding task inputs and migrate to property API where needed to ensure migrateToBazel provides fast local development loop.

1. Separate resolve dependencies into separate aggregating task and make it cacheable
2. Establishing input and output for generator and formatters.
3. Make artifact pinning cacheable by making WORKSPACE as the input and maven_install.json as the output.