The default prefix is /usr/local on macOS
and /home/linuxbrew/.linuxbrew on Linux.

Fixes #1952.

Return `opt_prefix` if it exists and `prefix` is not called from within
the same formula's `install` or `post_install` methods. Otherwise, fall
back to the existing functionality.

This avoids the need to use `opt_prefix` etc. everywhere and generally
means we don't expose an implementation detail (i.e. the full Cellar
path) to dependents that have a habit of hard-coding it.

Closes Homebrew/homebrew#42354.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Rationale: everyone thinks it points to the currently installed prefix, not the prefix of the formula that is newest and maybe not installed.

The previous behavior was unintentional and evidence proves it is not intuitive either.

I anticipate no regressions. And hope I am right!

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

The code was sucking. To the extent that maintenance was hard. It's a lot
easier to work with code that is sensibly split at sensible boundaries. So
now it is more like that.

But the refactor is minimal. Because we don't want you to have more merge
hell than absolutely necessary.

If you merge you will need to pay attention to brew.h.rb (as it is deleted)
and bin/brew (as command logic is gone). It will be painful, but you will just
have to help git out by moving any changes around manually.

Note compatibility.rb. It ensures that any function renames or removals don't
break anything. We're pretty serious about backwards compatibility. And that's
because we encourage you to hack around with the innards. And we couldn't do
that if we would then just make stuff disappear behind your back.