This allows the use case where installation of a plugin can simply say
to add `$GOPATH/bin/foo` to your terraformrc and the user can do that
verbatim. Additionally terraformrc files become portable for certain
environments which are self-contained.

I was hesitant at first about this because it diverges the syntax a bit
from our standard interpolation syntax. However, due to the special
nature of terraformrc and the strong use cases cited I'm okay with this.