Rather than try to modify all the hundreds of calls to the temp helper
functions, and cleanup the temp files at every call site, have all tests
work within a single temp directory that is removed at the end of
TestMain.
As part of the 0.10 core/provider split we moved this provider, along with
all the others, out into its own repository.
In retrospect, the "terraform" provider doesn't really make sense to be
separated since it's just a thin wrapper around some core code anyway,
and so re-integrating it into core avoids the confusion that results when
Terraform Core and the terraform provider have inconsistent versions of
the backend code and dependencies.
There is no good reason to use a different version of the backend code
in the provider than in core, so this new "internal provider" mechanism
is stricter than the old one: it's not possible to use an external build
of this provider at all, and version constraints for it are rejected as
a result.
This provider is also run in-process rather than in a child process, since
again it's just a very thin wrapper around code that's already running
in Terraform core anyway, and so the process barrier between the two does
not create enough advantage to warrant the additional complexity.
The -plugin-dir option lets the user specify custom search paths for
plugins. This overrides all other plugin search paths, and prevents the
auto-installation of plugins.
We also make sure that the availability of plugins is always checked
during init, even if -get-plugins=false or -plugin-dir is set.
Previously we had a "getProvider" function type used to implement plugin
fetching. Here we replace that with an interface type, initially with
just a "Get" function.
For now this just simplifies the interface by allowing the target
directory and protocol version to be members of the struct rather than
passed as arguments.
A later change will extend this interface to also include a method to
purge unused plugins, so that upgrading frequently doesn't leave behind
a trail of unused executable files.
