Use parallel enumeration for sources

[jonabc]

Following #207 and #204, this updates a few more sources to use parallel enumeration

1. Cabal

• when obtaining full package information from package ids
• when recursively obtaining indirect package dependencies
• tried when getting package ids for initial set of dependencies found in cabal files, which slowed things down in local testing

2. Go < 1.11
   • when getting package info for all dependencies listed in root_package["Deps"]
   • cannot use when getting package version. this can potentially make many git cli calls, but by necessity of being in the target packages git repo I need to Dir.chdir package_directory, where Dir.chdir is not threadsafe.
3. Manifest
   • when getting package versions which potentially makes many git cli calls

Speed gains observed from CI

	Cabal	Go	Manifest
Absolute	1-3s	25-32s	0s - these were already pretty fast at 3s
Relative	10-30%	33-39%	0%