Wrong subversion dependency when fetch.type is svn on Catalina

[ryandesign (Ryan Carsten Schmidt)]

When fetcy.type is svn, MacPorts base declares a dependency on bin:svn:subversion. On Catalina with only the command line tools installed, /usr/bin/svn exists but is a stub that doesn't do anything:

:info:fetch svn: error: The subversion command line tools are no longer provided by Xcode.

We already change the dependency to port:subversion on old OS versions whose svn can't connect to secure servers anymore. We should also do this when on a new system with a stub svn.