As well as constraining plugins by version number, we also want to be
able to pin plugins to use specific executables so that we can detect
drift in available plugins between commands.
This commit allows such requirements to be specified, but doesn't yet
specify any such requirements, nor validate them.
As we add support for versioned providers, it's getting more complex to
track the dependencies of each module and of the configuration as a whole,
so this new package is intended to give us some room to model that
nicely as a building block for the various aspects of dependency
management.
This package is not responsible for *building* the dependency data
structure, since that requires knowledge of core Terraform and that would
create cyclic package dependencies. A later change will add some logic
in Terraform to create a Module tree based on the combination of a given
configuration and state, returning an instance of this package's Module
type.
The Module.PluginRequirements method flattens the provider-oriented
requirements into a set of plugin-oriented requirements (flattening any
provider aliases) giving us what we need to work with the plugin/discovery
package to find matching installed plugins.
Other later uses of this package will include selecting plugin archives
to auto-install from releases.hashicorp.com as part of "terraform init",
where the module-oriented level of abstraction here should be useful for
giving users good, specific feedback when constraints cannot be met.
A "reason" is tracked for each provider dependency with the intent that
this would later drive a UI for users to see and understand why a given
dependency is present, to aid in debugging sticky issues with
dependency resolution.
