Plugin does not remap symbolMapping entry when merging reflections

[rbuckton]

When the plugin merges external module reflections it calls CommentsPlugin.removeReflection, which deletes the entry in project.symbolMapping and can break resolution.

Consider tracking the symbol id in https://github.com/christopherthielen/typedoc-plugin-external-module-name/blob/master/plugin.ts#L75 via a call to context.getSymbolID(node.symbol), and updating the entry in project.symbolMapping before calling CommentsPlugin.removeReflection on https://github.com/christopherthielen/typedoc-plugin-external-module-name/blob/master/plugin.ts#L132.