Fix class not found exception without Subversion plugin

[pfeuffer]

Fixes https://issues.jenkins.io/browse/JENKINS-64985

A ClassNotFoundException was thrown while creating namespace jobs, when the Subversion plugin is not installed. The stack trace looked something like this:

java.lang.ClassNotFoundException: jenkins.scm.impl.subversion.SubversionSCMSource
        at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1387)
        at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1342)
        at jenkins.util.AntClassLoader.loadClass(AntClassLoader.java:1089)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
Caused: java.lang.NoClassDefFoundError: jenkins/scm/impl/subversion/SubversionSCMSource
        at java.base/java.lang.ClassLoader.defineClass1(Native Method)
        at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1017)
        at jenkins.util.AntClassLoader.defineClassFromData(AntClassLoader.java:1155)
        at hudson.ClassicPluginStrategy$AntClassLoader2.defineClassFromData(ClassicPluginStrategy.java:715)
        at jenkins.util.AntClassLoader.getClassFromStream(AntClassLoader.java:1326)
        at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1377)
        at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1342)
        at jenkins.util.AntClassLoader.loadClass(AntClassLoader.java:1089)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
        at com.cloudogu.scmmanager.scm.ScmManagerNavigator.visitSources(ScmManagerNavigator.java:142)
        at jenkins.branch.OrganizationFolder.computeChildren(OrganizationFolder.java:542)
        at com.cloudbees.hudson.plugins.folder.computed.ComputedFolder.updateChildren(ComputedFolder.java:278)
        at com.cloudbees.hudson.plugins.folder.computed.FolderComputation.run(FolderComputation.java:165)
        at jenkins.branch.OrganizationFolder$OrganizationScan.run(OrganizationFolder.java:1033)
        at hudson.model.ResourceController.execute(ResourceController.java:97)
        at hudson.model.Executor.run(Executor.java:429)

This was due to a reference to a classes only available in the Subversion plugin in ScmManagerSvnSource, and that class in turn was a part of the public API of ScmManagerSvnSourceBuilder due to the generic. But this class is used in ScmManagerNavigator.ScmManagerSvnSourceBuilder and therefore has to be loaded when a job shall be created.

By using SCMSource in the generic instead of the special implementation of the Subversion plugin, we can remove this class from the public API, and therefore this class no longer needs to be loaded unless it is needed.

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