Auxiliary sbt commands for building individual
stdlib packages.
The commands check if the engine distribution was built at least once,
and only copy the necessary package files if necessary.
So far added:
- `buildStdLibBase`
- `buildStdLibDatabase`
- `buildStdLibTable`
- `buildStdLibImage`
- `buildStdLibGoogle_Api`
Related to [#182014385](https://www.pivotaltracker.com/story/show/182014385)
Implements https://www.pivotaltracker.com/story/show/181266184
### Important Notes
Changed example image download to only proceed if the file did not exist before - thus cutting on the build time (the build used to download it _every_ time - which completely failed the build if network is down). A redownload can be forced by performing a fresh repository checkout.
