Improve handling of 'dependencies' argument

[Enchufa2]

#32 added this feature to the default path, but not to the fast path, which currently just ignores this argument. This means that Suggests or Enhances can only be installed via the default path. There are two possibilities:

1. Handle this argument in the fast path too.
2. If there is some limitation with the above, document this behavior, and emit an informative warning if this argument is specified with bspm.version.check set to FALSE.