brew.sh: reorder to speedup shellenv.

Reorder and restructure this file to make it as fast as possible for
e.g. `brew --prefix`. Once we have that, also give `brew shellenv`
short-circuit command execution to make it as fast as possible.

On my machine (with a warm cache) this gets:

```
brew shellenv  0.01s user 0.03s system 100% cpu 0.038 total
```

Compared to:

```
brew shellenv  0.09s user 0.19s system 100% cpu 0.277 total
```

before this commit.