Require references from typealiases to be const (#516)

This adds a language change that requires references from typealiases
to the enclosing module to be `const`.

This is required because typealiases are not late-bound.

Rationale is laid out in SPICE-0007.

Also:
* Update documentation to reflect new rules.
* Fix `Project.pkl`; mark method `const` to not break said rule.

stdlib/Project.pkl

@@ -221,7 +221,7 @@ function newInstance(enclosingModule: Module): Project = new {
   projectFileUri = reflect.Module(enclosingModule).uri
 }
 
-local hasVersion = (it: Uri) ->
+const local hasVersion = (it: Uri) ->
   let (versionSep = it.lastIndexOf("@"))
     if (versionSep == -1) false
     else let (version = it.drop(versionSep + 1))