REPLCompletions: allow completions for `[import|using] AAA: xxx` (#54719)
When an input line like `[import|using] AAA: xxx` is entered, if `AAA`
is already loaded in the REPL session, it will now autocomplete to the
names available in `AAA` that start with `xxx`.
For example, it will autocomplete `using Base.Experimental: @ov|` to
`using Base.Experimental: @overlay`.
- fixes #23374