Consolidate the handling of which directories need to exist and which
need to be writable. Additionally, add a fatal check for formula
installations to ensure that any directories that need to be writable
are so before attempting an installation.
Fixes#4626.
- Note that Homebrew Cask issues should be filed elsewhere.
- Make `brew`/homebrew-core issue differential in simpler language
- Tell people to fix all `brew doctor` issues
- Refer to the issue checklist and make it even more clear and prominent
that we may close it out immediately if they don't use it.
- Note that proposed new issues should be widely relevant.
- Don't tell people they need to `brew cask install java`; we handle
this in most places with a Java requirement and probably want issues
filed where we don't.
- Remove reference to Homebrew Dupes and replace with reference to
Homebrew Cask to try and get people filing stuff there instead.
Fixes#2608
