Add support for macOS Big Sur

[rotu]

No description provided.

[cottsay]

Two thoughts:
1. None of the hundreds of osx rosdep keys are going to work on a platform that detects the OS as macos, regardless of whether the same OSDetector class is used. You should use an alias, as is done for derivatives of Ubuntu and RHEL.
2. I'm not thrilled to see a mixture of refactor changes mixed in with the change you're proposing. If you believe moving stuff into that class provides value, I think you should open a separate PR, and we'll discuss whether it is worth touching the lines and complicating a git blame process.

[cottsay]

None of the hundreds of osx rosdep keys are going to work on a platform that detects the OS as macos, regardless of whether the same OSDetector class is used. You should use an alias, as is done for derivatives of Ubuntu and RHEL

Nix that, the aliasing happens in rosdep, not here. Mea culpa.

[rotu]

Everything past Sierra is macOS, and Big Sur is not even macOS 10. Feel free to propose a better fix or only pull some of these changes.

Nevermind I'll submit issues instead

[rotu]

extracted to #209 and #210