Move addrs/ to internal/addrs/

This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.

If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.

internal/addrs/doc.go

@@ -0,0 +1,17 @@
+// Package addrs contains types that represent "addresses", which are
+// references to specific objects within a Terraform configuration or
+// state.
+//
+// All addresses have string representations based on HCL traversal syntax
+// which should be used in the user-interface, and also in-memory
+// representations that can be used internally.
+//
+// For object types that exist within Terraform modules a pair of types is
+// used. The "local" part of the address is represented by a type, and then
+// an absolute path to that object in the context of its module is represented
+// by a type of the same name with an "Abs" prefix added, for "absolute".
+//
+// All types within this package should be treated as immutable, even if this
+// is not enforced by the Go compiler. It is always an implementation error
+// to modify an address object in-place after it is initially constructed.
+package addrs