don't warn when modules have the name of something in Base

[StefanKarpinski]

The classic example come from using Gadfly:

Warning: using Base.Stat in module Stat conflicts with an existing identifier.

This could possibly be fixed by binding the name of the module after doing the automatic using Base in a new module.

[JeffBezanson]

I'm not sure that warning is necessary at all. Why should these behave differently:

julia> module Foo
         symv = 0
         using Base.BLAS
       end
Warning: using BLAS.symv in module Foo conflicts with an existing identifier.

vs:

julia> module Foo                
         using Base.BLAS
         symv = 0
       end
# silence

[StefanKarpinski]

I can get on board with that too.

[JeffBezanson]

The history of this is #2054.