Here is part of the sketch showing the #include for <Adafruit_PWMServoDriver.h>
When a library contains the word “Adafruit”, you should start by assuming it’s one of Adafruit’s libraries, and look under their GitHub account for it.
If you do a search across all of GitHub for the library, you’ll likely find derivative projects, which are less likely to be the dependency of your sketch than the original.
The Adafruit_Sensor is a perfect example.
The zip file within .github is named Adafruit_Sersor-master.zip which is what is downloaded.
Within the .zip file is the folder
That’s just how GitHub works. There’s nothing Adafruit’s “library manager” can do about it. GitHub appends the Git ref of the repository download to the end of the repository name. You downloaded the tip of the master branch, so “master” was appended. In almost all cases, it makes no difference what the folder is named, so you’re free to leave the -master on the folder name.