This is a slightly different approach than we used to take for this
option: rather than disabling the installer and causing all future
commands to look elsewhere for plugins, we'll now leave the installer
enabled by constrain it to only look at the given directories.
This is overall simpler because it doesn't require any special tracking
of the plugin directories for subsequent commands. Instead, the selections
file generated by the installer will record the versions it selected from
the specified directories, and we'll link them in to the local cache just
as we would normally so that other commands don't need to do anything
special to select the right plugins in either case.
Back when we first introduced provider versioning in Terraform 0.10, we
did the provider version resolution in terraform.NewContext because we
weren't sure yet how exactly our versioning model was going to play out
(whether different versions could be selected per provider configuration,
for example) and because we were building around the limitations of our
existing filesystem-based plugin discovery model.
However, the new installer codepath is new able to do all of the
selections up front during installation, so we don't need such a heavy
inversion of control abstraction to get this done: the command package can
select the exact provider versions and pass their factories directly
to terraform.NewContext as a simple static map.
The result of this commit is that CLI commands other than "init" are now
able to consume the local cache directory and selections produced by the
installation process in "terraform init", passing all of the selected
providers down to the terraform.NewContext function for use in
implementing the main operations.
This commit is just enough to get the providers passing into the
terraform.Context. There's still plenty more to do here, including to
repair all of the tests this change has additionally broken.
These new functions allow command implementations to get hold of the
providercache objects and installation source object derived from the
current CLI configuration.
Martin Atkins