Pkg.generate should use stderr. (#2870)

Author: fredrikekre

src/generate.jl

@@ -1,6 +1,6 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
-function generate(path::String; io::IO=stdout_f())
+function generate(path::String; io::IO=stderr_f())
     base = basename(path)
     pkg = endswith(lowercase(base), ".jl") ? chop(base, tail=3) : base
     Base.isidentifier(pkg) || pkgerror("$(repr(pkg)) is not a valid package name")