Config files are cached by file path to avoid needing to load the same config file twice. However, configs provided by plugins all have the same file path (the index file of the plugin). This created a bug when loading multiple configs for the same plugin where only the highest-precedence config from that plugin would be loaded. All other configs from that plugin would be considered identical by the cache, so they would all end up getting pulled from the cache as the same config.

This commit updates the caching logic to use the config full name as a cache index. This is still the file path when resolving a config from the filesystem, but it is the unique plugin config identifier (e.g. 'plugin:foo/node-config') when resolving a plugin config.