Do this instead of appending the `HOMEBREW_PREFIX` as it's more
reliable at finding e.g. keg-only and requirement PATH additions. It's
not ideal as it overrides some environment filtering but the only real
alternative is recreate the entire build environment by shelling out to
`build.rb` in `formula_installer`.
Improve the messaging around `brew install` when there's a possible user
action such as an `upgrade` or `link` and don't tell people to
`install --force` when it's unnecessary.
While I did this, tweak the output and function usage in a couple of
related places.
Some example output before this change:
```
Warning: openssl is a keg-only and another version is linked to opt.
Use `brew install --force` if you want to install this version
Warning: mysql@5.6 is a keg-only and another version is linked to opt.
Use `brew install --force` if you want to install this version
Warning: analog-6.0_1 already installed
Warning: bash-completion@2-2.5 already installed, it's just not linked.
```
Some example output after this change:
```
Error: openssl 1.0.2k is already installed
To upgrade to 1.0.2l, run `brew upgrade openssl`
Warning: mysql@5.6 5.6.36_1 is already installed
Warning: analog 6.0_1 is already installed
Warning: bash-completion@2 2.5 is already installed, it's just not linked.
You can use `brew link bash-completion@2` to link this version.
```
Only likely to kick in when environment filtering is enabled.
Otherwise we need to tediously add a dramatic number of PATHs to tests
or recurse through the runtime formulae dependencies and add all them.
CC @ilovezfs
Also, don't delete them after that. This means that `brew postinstall`
becomes a way to easily reinstall configuration files for any formula
without needing any changes to any bottles or requiring a reinstall.
Hide these tokens to avoid malicious subprocesses e.g. sending them
over the network. Also, support using these tokens with environment
filtering and clear `HOMEBREW_PATH` from subprocesses to stop them
sniffing it. Finally, use `HOMEBREW_PATH` to detect Homebrew’s user’s
PATH for e.g. `brew doctor` etc.
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.
