Why I have introduced this change is so that we may be able to produce
Juju binaries for darwin/arm64. But the ultimate point of this work is
to make the changes necessary so a Juju developer can work on Juju from
any Darwin machine with either x86 or arm64 instruction sets.

The changes being done in this PR are:

- The first changed introduced is 3 new Makefile variables that define
  the set of platforms to compile various aspects of Juju for. These
  variables can be defined by the user from the command line to have
  make build Juju for multiple platforms. If not supplied it will
  default to the values of GOOS & GOARCH.

- Build targets for all of the Juju packages have been broken out into
  their own separate targets. Previously one target would do all the
  building of Juju packages. Having seperate targets allows for a
  variable list platforms to build packages for to be established. It
  also provides a way to build specific packages and targets without the
  building over everything else.

  make build still exists and executes.

- Install targets for all of the Juju packages. This is the same change
  as build and now you can type make juju jujuc or make install.

- Updates the operator-image build target to also build juju for only
  the packages that are needed for the OCI image.

- Update of the Docker image so that the context it operates from is
  know longer a context directory with pre package binaries. It now
  expects the context has directories available for each arch that the
  Docker image is being built for.

- Have added a new version cmd that can be run with
  go run version/helper/main.go that will print the current version of
  juju.

- Moved .PHONY targets above their respective targets for better
  tracking and consistency. Before this the targets were not being
  maintained with their .PHONY status.