Plugins: support plugin URLs with dependencies + anonymous module definitions

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Currently, plugins that use the standard anonymous module definition style will fail to load relative dependencies. They will be loaded by their URL and require.js will not treat the relative dependency names as module names, but as URLs.

A related issue is r.js-built plugin dependencies not having the right module URL on their module map object.

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Obsolete as of 26c3afe.

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Going to recommend using extplug-cli instead