Add a new package, `codegen/importer`, that can generate definitions for
resource states in PCL or TS/Python/C#/Go. The pipeline is relatively
simple: given a list of resource states, generate a PCL program in
memory, bind it, and pass it to the language-specific code generator.
This builds upon the existing PCL IR, and can be used with the currently
supported code generators.
Related to #1635.
Instead of requiring a plugin host for package loading in the HCL2
binder, define a much narrower interface that exposes the ability to
fetch the schema for a package at a specific version. This interface is
defined in the schema package, which also exposes a caching loader that
is backed by provider plugins.
These changes also add some convenience methods to `*schema.Package` for
fast access to particular resources and functions.
Related to #1635.
