rustpkg Initialize BuildContext/WorkCache later in the build process

[nielsle]

The BuildContext mainly consists of WorkCache, and that is not needed for commands like "rustpkg init". Therefore it would be nice to initialize BuildContext later in the build process.

Right now "rustpkg init" calls a method on the trait CtxMethods which is implemented by BuildContext, so you cannot run "rustpkg init" without initializing BuildContexts, so we should start out replacing the methods of CtxMethods with functions.

This issue keeps track of the work and resulting cleanups.

PS: I started out trying to do everything in one pull request, but it became a little too long so I retracted it again #11379