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-extension-api/src/main/java/com/google/gerrit/extensions/annotations/PluginData.java

@@ -27,11 +27,15 @@ import java.lang.annotation.Target;
  * <p>
  * A plugin or extension may receive this string by Guice injection to discover
  * a directory where it can store configuration or other data that is private:
+ * <p>
+ * This binding is on both {@link java.io.File} and {@link java.nio.file.Path},
+ * pointing to the same location. The {@code File} version should be considered
+ * deprecated and may be removed in a future version.
  *
  * <pre>
  * {@literal @Inject}
- * MyType(@PluginData java.io.File myDir) {
- *   new FileInputStream(new File(myDir, &quot;my.config&quot;));
+ * MyType(@PluginData java.nio.file.Path myDir) {
+ *   this.in = Files.newInputStream(myDir.resolve(&quot;my.config&quot;));
  * }
  * </pre>
  */