bash-completion: look: rewrite completion logic

bash-completion: look: rewrite completion logic

look(1) opens /usr/share/dict/words as dictionary by default. Doing TAB
completion for look(1) without the dictionary installed will result in
popups like

	$ look look: /usr/share/dict/words: No such file or directory

messing up the terminal.

As /usr/share/dict/words is not shipped by quite a few distros nowadays
and it should not be listed as a hard dependency of util-linux, we make
sure the dictionary exists before calling look(1) when doing completion,
avoiding these error messages.

When completing the word without anything specified on the command line,
alphabets and numbers instead of all words are listed as a hint, for
better interacting experience.

Signed-off-by: Yao Zi <ziyao@disroot.org>