Needed for `brew diy`.

Fixes #9036

The way we currently handle @-versioned formulae linking is pretty
labourius:
- it requires extensive use of `link_overwrite` to avoid the `link`
  stage failing on certain install/upgrade scenarios
- we teach people to use `brew link --force` whenever they wish to
  link a versioned formulae when it's pretty obvious what's expected
  in that situation

Instead, let's:
- automatically unlink other versioned formulae when linking a
  versioned formula (either through `brew link` or `install`/`upgrade`
  /`reinstall`)
- notify the user what we've done (with the same messaging as if
  they had run `brew link` manually)

Use active voice when applicable, better verbs & adjectives, preposition usage, proper tense, etc.

We've not seen complaints about these and they are the default behaviour
on High Sierra and Mojave so it's easier to apply them consistently
everywhere.

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

These are a bit easier to follow and have been recently improved.

Inspired by conversation in #4441.

The change in #4441 broke the handling of the `elsif`s due to the
change in logic. As every block here has a `next` there's no need to do
an `elsif` in here at all. Additionally, reorder the conditions in here
so you get an appropriate message depending on what you're trying to do.
Finally, tweak some of the messaging to remove things that are ignored
and tell people correct commands to run to link things.

People are getting in the habit of force-linking things like `zlib` to
fix linking/include issues on Mojave (which doesn't install headers to
`/usr/include` by default). This way lies madness so encourage people to
instead pass the correct compiler flags instead.

Deprecate more methods. Internal APIs have been verified to be unused
elsewhere and removed. External APIs have had deprecation methods added.
Existing deprecations have been either upgraded to produce warnings or
no longer deprecated and the reasoning documented.

Commit 4cae6a72 introduced the message,
but it printed the wrong path, e.g. for `brew link sqlite`:

```
If you need to have this software first in your PATH instead consider running:
  echo 'export PATH="/usr/local/opt/sqlite:$PATH"' >> ~/.zshrc/bin
```

where `/bin` is appended at the end, but should be inserted before
`:$PATH`: `echo 'export PATH="/usr/local/opt/sqlite/bin:$PATH"' >> ~/.zshrc`.
This patch fixes that and updates a test to verify it.

If people are force-linking keg-only things they should probably be told
that they can add the opt prefix to their PATH instead.