Update polygerrit_plugin() build rule
Update polygerrit_plugin() build rule

Add support for .js based plugins, custom plugin_name to prevent
conflicts with server-side plugins with the same name.

Here's an example:
https://chromium-review.googlesource.com/c/infra/gerrit-plugins/landingwidget/+/1295049

So what we want to achieve is to have:

  gerrit_plugin(
    name = "landingwidget",
  [...]

and

  polygerrit_plugin(
    name = "landingwidget_ui",
    plugin_name = "landingwidget",
  [...]

and produce the following final artifacts:

  landingwidget.jar
  landingwidget.js

Change-Id: Ic9c68809cbaaf4650301d98e56d7c1c76e008251

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