When building a swift object, we emit a partial swiftmodule and swiftdoc
that must be merged at the end.  However, in order to do that, we need
to enumerate the swiftmodules and swiftdocs.  As a result, the path must
be known to CMake.  Rather than hardcoding the rules into CMake, create
a source property that we can query.  This will allow us to create a
final placeholder to emit the merge rule.

Issue: #18800