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/brew.completion.bash

@@ -1,13 +1,27 @@
-if which brew >/dev/null 2>&1; then
-  BREW_PREFIX=$(brew --prefix)
-  if [ -f "$BREW_PREFIX"/etc/bash_completion.d/brew ]; then
-    . "$BREW_PREFIX"/etc/bash_completion.d/brew
-  elif [ -f "$BREW_PREFIX"/Library/Contributions/brew_bash_completion.sh ]; then
-    . "$BREW_PREFIX"/Library/Contributions/brew_bash_completion.sh
-  elif [ -f "$BREW_PREFIX"/completions/bash/brew ]; then
-    # For the git-clone based installation, see here for more info:
-    # https://github.com/Bash-it/bash-it/issues/1458
-    # https://docs.brew.sh/Shell-Completion   
-    . "$BREW_PREFIX"/completions/bash/brew
-  fi
+#!/usr/bin/env bash
+
+# Load late to make sure `system` completion loads first
+# BASH_IT_LOAD_PRIORITY: 375
+
+if [[ "$(uname -s)" != 'Darwin' ]] ; then
+  _log_warning "unsupported operating system - only 'Darwin' is supported"
+  return 0
+fi
+
+# Make sure brew is installed
+_command_exists brew || return 0
+
+BREW_PREFIX=${BREW_PREFIX:-$(brew --prefix)}
+
+if [[ -r "$BREW_PREFIX"/etc/bash_completion.d/brew ]] ; then
+  source "$BREW_PREFIX"/etc/bash_completion.d/brew
+
+elif [[ -r "$BREW_PREFIX"/Library/Contributions/brew_bash_completion.sh ]] ; then
+  source "$BREW_PREFIX"/Library/Contributions/brew_bash_completion.sh
+
+elif [[ -f "$BREW_PREFIX"/completions/bash/brew ]] ; then
+  # For the git-clone based installation, see here for more info:
+  # https://github.com/Bash-it/bash-it/issues/1458
+  # https://docs.brew.sh/Shell-Completion
+  source "$BREW_PREFIX"/completions/bash/brew
 fi