Check for conflicting @ccallable name before JIT registration

[topolarity]

Fixes #54878. For pkgimages with conflicting @ccallable definitions:

$ cat Foo/src/Foo.jl
module Foo
Base.@ccallable function foo()::Cint
    return 0
end
end
$ cat Bar/src/Bar.jl
module Bar
Base.@ccallable function foo()::Cint
    return 0
end
end

This turns the existing segfault into a warning, as on 1.9:

julia> using Foo, Bar
WARNING: @ccallable was already defined for this method name