bash: depend on gettext

[carlocab]

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This currently vendors its own, but we should use the gettext formula
instead.

Also:

• Instead of hardcoding a bunch of paths in DEFAULT_LOADABLE_BUILTINS_PATH,
  let's just set the right -rpath flag instead. This is sufficient to
  help bash find loadable modules inside lib/bash.
• Move a bunch of files in lib/bash into better locations.