brew.sh: reorder to speedup shellenv.

[MikeMcQuaid]

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.

Credit to @metaColin for giving me this idea in #8846 (comment)

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