ModuleAvailableFeature broken for modules with side-effects

ModuleAvailableFeature uses the following code to check for the presence of a module:

    def _probe(self):
        try:
            self._module = __import__(self.module_name, {}, {}, [''])
            return True
        except ImportError:
            return False

This fails for 'bzrlib.plugins.svn' with:

    self._module = __import__(self.module_name, {}, {}, [''])
  File "/home/vila/.bazaar/plugins/svn/__init__.py", line 140, in <module>
    help="Access using the Subversion smart server tunneled over SSH.")
  File "/home/vila/src/bzr/trunk/bzrlib/transport/__init__.py", line 135, in register_transport_proto
    transport_list_registry.register_transport(prefix, help)
  File "/home/vila/src/bzr/trunk/bzrlib/transport/__init__.py", line 123, in register_transport
    self.register(key, [], help)
  File "/home/vila/src/bzr/trunk/bzrlib/registry.py", line 117, in register
    raise KeyError('Key %r already registered' % key)
KeyError: "Key 'svn+ssh://' already registered"

Because the plugin has already been imported and its init code has already run.

- self._module = __import__(self.module_name, {}, {}, [''])
+ exec 'import %s' % (self.module_name)
+ self._module = sys.modules[self.module_name]

fixes the problem.

I'll submit patch.