using creates explicit bindings inconsistently

[LilithHafner]

julia> module M1 module M2 end end
Main.M1

julia> using .M1.M2

julia> using Statistics

julia> names(Main, imported=true)
5-element Vector{Symbol}:
 :Base
 :Core
 :M1
 :Main
 :Statistics

julia> module M3
           using Statistics
       end
Main.M3

julia> names(M3, imported=true)
1-element Vector{Symbol}:
 :M3

This is due to

julia/src/toplevel.c

Lines 771 to 775 in b354ce7

	if (m == jl_main_module && name == NULL) {
	// TODO: for now, `using A` in Main also creates an explicit binding for `A`
	// This will possibly be extended to all modules.
	import_module(m, u, NULL);
	}

Which I think was added 7 years ago in #25256

[LilithHafner]

A more significant consequence of this is:

julia> Statistics = 4
4

julia> using Statistics
WARNING: using Statistics.Statistics in module Main conflicts with an existing identifier.
ERROR: cannot declare Main.Statistics constant; it already has a value

julia> module M4
       Statistics = 5
       using Statistics
       end
WARNING: using Statistics.Statistics in module M4 conflicts with an existing identifier.
Main.M4