Fix ?(#TAB method search name exploration (#52555)
Fix #52551.
This PR ensures that a `SomeModule.?(...#TAB` completion can only
suggests method `foo` such that `SomeModule.foo` exists (by checking
`isdefined(SomeModule, :foo)`). This is equivalent, I believe, to the
initial implementation of https://github.com/JuliaLang/julia/pull/38791,
less the bug.
Now that we have #51345, we may want to relax the above condition
somewhat to include public names present in modules loaded into
`SomeModule`, so that, for instance, a direct completion of `?(` would
include `@assume_effects`. This could be good for method exploration
because even though typing `@assume_effects` with no qualification in
`Main` will error, the error now includes the helpful message
```
Hint: a global variable of this name also exists in Base.
```
But that can wait for a later PR anyway, this one is just the bugfix.
The bug mentioned at
https://github.com/JuliaLang/julia/issues/52551#issuecomment-1858543413
dates from the initial #38791 so this could be backported as far back as
v1.8.
---------
Co-authored-by: Shuhei Kadowaki <aviatesk@gmail.com>
