There in no tool available in iPhoto or Photos for merging libraries. You can only import photos and folders from within Photos app. The consolidate feature works different from import and does not allow you to "Consolidate" multiple libraries. The iPhoto and Photos app allows you to keep originals in folders and keep track of changes and metadata. Those photos will now store in iCloud Photo Library and keeping them outside of the "Photos" library also makes the library less portable.
If you want to consolidate your libraries in Photos for OS X, then choose one or more photos from the photo library and click "File" menu and then select "Consolidate" option. Photos app asks for permission to access locations outside its library in order to consolidate all photos in its library. Photos app needs permission to access this media. Now choose the folder named "Pictures" and tap "Ok" button.
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