.sln.metaproj.tmp written to disk too soon

[radical]

This writes the metaproj for the solution to disk here. But after this point there are changes made to the project, eg. a .csproj.metaproj reference might be added like in #1957 .
Moving that to around here fixes it. But I'm a little unsure because of this comment:

            // For debugging purposes: some information is lost when evaluating into a project instance,
            // so make it possible to see what we have at this point.