We've now upgraded the go-slug dependency to a newer version that fixed
the bug that this FIXME comment was referring to, so we can now write the
source address correctly and remove the note.
Instead of the temporary hack of hard-coding the built-in "terraform"
provider, it's now possible to declare configurations for arbitrary
providers and assign them to the provider configuration slots of the root
module of each component.
This means it's valid to write references like "component.foo" in
expressions in a stack configuration, where the result is built from the
planned output values of the root module of each component instance.
For embedded stacks we intentionally designed the language to require
explicit types on all output values so that we'd still be able to do type
checking even when we can't calculate the output values yet. Unfortunately
we can't do the same trick for components because the main Terraform
module language treats output values as dynamically-typed and so an
output value we can't evaluate yet could have literally any type.
Therefore we concede and just return cty.DynamicVal in most failure cases
here; perhaps a future edition of the main Terraform language can improve
this by requiring explicitly-typed output values there too.
This is the absolute bare-minimum integration with the main Terraform
language runtime to plan changes for a component. It's intentionally not
yet very robust and limited only to the built-in Terraform provider
because we're just trying to get the bones in place for seeing stack
planning working, rather than trying to build something complete enough
to ship.
This currently only deals with the resource instance changes that get
planned and doesn't expose the root module's output values for use
elsewhere in the stack configuration. Output value handling will follow
in subsequent commits.
