Document package.sh BUILD_DEPS and deprecate dotenv module

Since the implementation of package.sh support, the use of the dotenv module isn't needed for use in .dkrc; it's already loaded by dk. And if you do need the dotenv command, you can just list it in BUILD_DEPS in package.sh.

This was implemented in cafecd4, and the documentation was done in 74f4414. So the dotenv module should be removable now.