split magit into multiple packages

Split (most) Magit libraries that depend on external binaries or scripts into separate packages.

I suggested this move at https://github.com/magit/magit/issues/845 as well as on the Magit mailing list. There were no objections and very little feedback. (Of topic: Are my uses of *at* and *on* correct here?)

The `magit` recipe lists `magit.info` and `dir` in `:files` but the files are not part of the tarball. What am I doing wrong?

Also included are `AUTHORS.md` (because we mention it in the library headers) and `README.md` (because the header `magit.el` isn't good enough yet. Most importantly it does not link to the FAQ on the wiki, which I will fix. But considering that we also have to include `AUTHORS.md` do you recommend against adding `README.md' in the tarball. I will fix the Commentary eventually.)

The `magit-flow.el` library that was previously part of the `magit` package, is intentionally not part of any package at this point. It is not very useful in its current form.