REPLCompletions: limit binding completions for explicitly specified modules

[aviatesk]

When completing a line where a module is explicitly accessed like A.xxx|, the current implementation of REPLCompletions considers all bindings available to A (e.g., including bindings exported by modules that A has usinged), but I think it would be more useful if the completion only includes the bindings defined by A. For example, the completion for the line LinearAlgebra.str| currently looks like

julia> LinearAlgebra.str
stride                      stride1
strides                     string
strip                       structured_broadcast_alloc

but in common cases, the desired completion would look like:

julia> LinearAlgebra.str
stride1 structured_broadcast_alloc

excluding bindings exported by Base and Core.

Following this observation, this commit makes REPL completions only show bindings defined by explicitly accessed module, unless shift is pressed. If shift is pressed, all bindings available in the module are shown as before.

[ericphanson]

does JSON.pa still complete to JSON.parse? since the function is defined in a submodule of JSON but is exported into the main module (and documented to be used as JSON.parse). For ExplicitImports.jl this kind of thing caused me to consider definitions in submodules to still be owned by the rootmodule (by default).