This fixes the run package's leaky abstraction; other packages no longer
need to know or care that run.Config even exists.
Note that since the various Steps now depend on having a non-nil pointer
to a run.Config, it's unsafe (or at least risky) to initialize them
directly. They should be created with their NewSTEPNAME functions. All
their fields are now private, to reflect this.
This change revealed more about how the system needs to work, so there
are some supporting changes:
* helm.upgrade and helm.help are now vars rather than raw functions.
This allows unit tests to target the "which step should we run"
logic directly by comparing function pointers, rather than having to
configure/prepare a fully-valid Plan and then infer the logic’s
correctness based on the Plan’s state.
* configuration that's specific to kubeconfig initialization is now part
of the InitKube struct rather than run.Config, since other steps
shouldn’t need access to those settings (particularly the secrets).
* Step.Execute now receives a run.Config so it can log debug output.
I'm vacillating about the choice to have separate Config structs in the
`helm` and `run` packages. I can't tell whether it's "good separation of
concerns" or "cumbersome and over-engineered." It seems appropriate at
the moment, though.
