Print warning on multiple installs of node-mapnik in node_modules tree

[flippmoke]

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If more than one node-mapnik ends up in the npm/node-modules dependency tree then:

• at best: font or datasource registration may unpredictably fail because the global caches only behave correctly if truly once instance is loaded in the process.
• at worst these issues only manifest in production and go unnoticed in local development
• also odd behavior can also occur when the duplicate node-mapnik versions are different and have different ABI - leading to either symbol errors at startup or crashes.

For these reasons it is critical that only one node-mapnik version is installed in the node_modules tree. This means that npm's behavior of non deduping duplicate modules (under various circumstances) is fundamentally incompatible with node-mapnik. Therefore we need to help users and developers ensure that only one node-mapnik is installed. This would help flag when:

• two dependencies are used that each depend on node-mapnik and unintentionally install different versions
• or the declaration of a top level node-mapnik does not work to dedupe the deps

[springmeyer]

re-wrote the above ticket with more detail - also refs https://github.com/mapbox/versioning-node-cpp (which could take language from this ticket)