There is no such plugin available to import from Photos App as it does not allow external editors to interact directly.
You will have to first export the images from Photos App and then, import into Lightroom.
You can refer this article for export :
Although I've never used the new Photos app I have fooled around with the older iPhoto app. In that app, the older one, when you imported image it Copied them to its own folder structure. Placing them in a folder called Masters that is buried inside the iPhoto PhotoLibrary.something Package folder. To get your images expand that Package by right clicking on it and select Show Package Contents. From that you should be able to find all your images. Just COPY them over to some other folder that YOU Created on your hard drive. From that new folder you created you can import them into LR.
Word of warning. Never use any App Apple Gives to you. They are all JUNK and want to take control of your files.
Thanks. I was able to successfully export from Photos into Lightroom, although it would be much nicer if there were a plugin. A tip for those out there also doing this, if you export into a temporary folder and then import into Lightroom, Lightroom just links to the images in the temporary folder. So don't delete the temporary folder thinking Lightroom made its own copy in its library.
IMHO this is very poor advice. Exporting from Photos will not be in the RAW format you used to take your images.
It is better to get your original Master images out of the bad Apple iPhoto and Photos apps and then import them into LR. So you may lose your original edits but if you shoot RAW you will have all those original files.
If you shoot in JPG only maybe it doesn't matter.
If you "export as original unmodified file" it will export the raw files and Lightroom will import them fine. I was actually having the opposite problem of Lightroom not importing the JPEG files.
Photos app allows you to export either original (RAW) or the version (jpg) so, choosing export as original is same as getting the masters folder.