Convert plugin loading code to use Path

All of the internal scanning/loading code now uses Path, as do the
plugin and data directories in SitePaths. For backwards compatibility,
we still inject the @PluginData directory as a File. We do break
backwards compatibility of the ServerPluginProvider extension point,
under the theory that there should be very few such extensions and
they can handle the pain.

Tested:
-Installed and reloaded cookbook plugin multiple times.
-Expanded dynamic *.ssh plugin loading in the cookbook, with
 monkey-testing.

Change-Id: I5212d248d9288b88d08731c9854b42196999aa73

gerrit-server/src/main/java/com/google/gerrit/server/config/SitePaths.java

@@ -35,8 +35,8 @@ public final class SitePaths {
   public final Path lib_dir;
   public final Path tmp_dir;
   public final File logs_dir;
-  public final File plugins_dir;
-  public final File data_dir;
+  public final Path plugins_dir;
+  public final Path data_dir;
   public final File mail_dir;
   public final File hooks_dir;
   public final File static_dir;
@@ -74,8 +74,8 @@ public final class SitePaths {
     etc_dir = p.resolve("etc");
     lib_dir = p.resolve("lib");
     tmp_dir = p.resolve("tmp");
-    plugins_dir = new File(site_path, "plugins");
-    data_dir = new File(site_path, "data");
+    plugins_dir = p.resolve("plugins");
+    data_dir = p.resolve("data");
     logs_dir = new File(site_path, "logs");
     mail_dir = etc_dir.resolve("mail").toFile();
     hooks_dir = new File(site_path, "hooks");