Bazel: Adapt docs for custom plugins with external deps

After flipping of
--incompatible_disallow_load_labels_to_cross_package_boundaries bit in
Bazel: [1], currently documented way to import external dependencies
from plugins that implement standalone build mode doesn't work any more.

This change documents an alternative approach how to make it work,
without abandoning of standalone build mode.

[1] bazelbuild/bazel#6408

Bug: Issue 10885
Change-Id: I7b1c4e9fa4f6ba8966efcde4cdee7d76d6f59d1c

Documentation/dev-build-plugins.txt

@@ -122,16 +122,25 @@ CUSTOM_PLUGINS_TEST_DEPS = [
 ----
 
 If the plugin(s) being bundled in the release have external dependencies, include them
-in `plugins/external_plugin_deps`. You should alias `external_plugin_deps()` so it
-can be imported for multiple plugins. For example:
+in `plugins/external_plugin_deps`. Create symbolic link from plugin's own
+`external_plugin_deps()` file in plugins directory and prefix the file with
+plugin name, e.g.:
 
 ----
-load(":my-plugin/external_plugin_deps.bzl", my_plugin="external_plugin_deps")
-load(":my-other-plugin/external_plugin_deps.bzl", my_other_plugin="external_plugin_deps")
+  $ cd plugins
+  $ ln -s oauth/external_plugin_deps.bzl oauth_external_plugin_deps.bzl
+  $ ln -s uploadvalidator/external_plugin_deps.bzl uploadvalidator_external_plugin_deps.bzl
+----
+
+Now the plugin specific dependency files can be imported:
+
+----
+load(":oauth_external_plugin_deps.bzl", oauth_deps="external_plugin_deps")
+load(":uploadvalidator_external_plugin_deps.bzl", uploadvalidator_deps="external_plugin_deps")
 
 def external_plugin_deps():
-  my_plugin()
-  my_other_plugin()
+  oauth_deps()
+  uploadvalidator_deps()
 ----
 
 [NOTE]