c54b717 caused a regression for pluginv1 on Windows, where extraneous
backslashes were added to BasePath of the plugin. For pluginv1 on windows,
BasePath() should return an empty string, since the plugin is fully aware
of the mount path. Also, unlike Linux where all paths are relative to "/",
Windows paths are dependent on system drives and mapped drives.
Fixes#30148
Signed-off-by: Anusha Ragunathan <anusha.ragunathan@docker.com>
