fix message lookup for core nodes in case of i18 locales directory ex…

[HiroyasuNishiyama]

…ists

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As reported by issue #1615, message catalogue lookup for core nodes become failed when i18n locales directory (nodes/core/<category>/locales) exists.

This is caused by loadNodeConfig function in red/runtime/nodes/registry/loader.js.
If a locales directory exists, loadNodeConfig try to look up message catalogue (.json) for nodes in modules under the directory. Because the combined message catalogue for core nodes are placed at different location, this look up fails and message catalogue is corrupted.

This PR fixes this problem by skipping message catalogue look up for core nodes in loadNodeConfig.

My proposal is to place info text of core nodes under nodes/core/locales (or separate message catalogue of core nodes from messages.json). But, this may needs more discussion. So, this PR expects to place info text under nodes/core/<category>/locales.

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[coveralls]

Coverage remained the same at 87.755% when pulling ce9643d on node-red-hitachi:fix-i18n-message-lookup into b378670 on node-red:master.