Added project-info flag to depmap goal.

Prints source roots, dependencies, and paths to libraries for targets and their dependencies in json format.
It can be run with: ./pants goal depmap /path/to/target --depmap-project-info

Adding the -format flag causes the output to return a single line of json:
./pants goal depmap /path/to/target --depmap-project-info --depmap-format

This functionality will be used for project import in the IntelliJ Pants plugin.

Testing Done:
Added tests for the goal (test_depmap.py), they pass

ci.sh runs successfully

Reviewed at https://rbcommons.com/s/twitter/r/600/