How to auto-import AVCHD video?
Try the free AVCHD to MOV Lite on the App Store to convert it to MOV format, and then import to Photos. This will remux the video (convert it without reprocessing/converting the audio or video) losslessly. 

(Hat tip to How can I convert .MTS file (AVCHD) to .mp4 by ffmpeg without re-encoding H264 video stream?
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