- manually `raise Errno::ENOENT` to ensure that a keg that doesn't exist
  isn't flagged as a system dependency.
- remove the inconsistent and incorrect summary messaging.

Otherwise `path.realpath` will raise `Errno::ENOENT` rather than the
expected `NotAKegError`.

Fixes https://github.com/Homebrew/brew/issues/9015

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)

We don't actually care what it points to because we shouldn't have
LinkedKegs around for aliases at all (unlike `opt` links).

Some refactoring while doing #7956.

We did these for opt symlinks already but not linkedkegs.

Anywhere we can use `blob/master` we can use `blob/HEAD` instead. This
will make life easier if we ever rename our default branch in future
(once/if Git and GitHub provides the necessary tooling to do so).

Fixes https://github.com/Homebrew/brew/issues/7110

- Only try to call `.match?` on strings that aren't nil.

- This also required auto-fixes for Layout/EmptyLinesAroundBlockBody and
  Layout/InconsistentIndentation once the auto-fixer had got rid of the
  "redundant begin"s.

This will work regardless of permissions.

- don't complain about "skipping" kegs when `brew cleanup` runs
  automatically
- better handle orphaned directories
- output a better error message when a keg cannot be removed

Fixes #4989
Fixes #5627
Fixes https://github.com/Homebrew/homebrew-core/pull/35958#issuecomment-458031634