Abstract external_plugin_deps() one level
Abstract external_plugin_deps() one level

This allows core plugins to be extended more easily. Right now, to add
additional core plugins one must copy the dependencies one level. This
is error prone and requires manual changes to the core's
external_plugin_deps.bzl when a plugin's dependencies changes. Instead,
this allows our core external_plugin_deps.bzl to become:

```
load(":gitiles/external_plugin_deps.bzl", "gitiles")

def external_plugin_deps():
  gitiles()
```

And repeat as needed for any additional core plugins we want to add.
Ideally we'd generate these automagically from `tools/bzl/plugins.bzl`
but this is a step in the right direction to that.

Change-Id: I70b99a83c1b382a082eeddf73017239ba1a29ab5

GerritCodeReview/plugins_gitiles