system completion nitpicks

- General cleanup of system completion
- Use homebrew's completion loading script instead
- Deprecate completions covered by system
- Adjust load priority of brew to happen after system

completion/available/crystal.completion.bash

@@ -1,11 +1 @@
-if which crystal >/dev/null 2>&1; then
-
-  if which brew >/dev/null 2>&1; then
-    BREW_PREFIX=$(brew --prefix)
-
-    if [ -f "$BREW_PREFIX"/etc/bash_completion.d/crystal ]; then
-      . "$BREW_PREFIX"/etc/bash_completion.d/crystal
-    fi
-  fi
-
-fi
+_log_warning 'Bash completion for "crystal" is now covered by "system". This completion can be disabled.'