Issue a warning when a file is included twice

[alexcrichton]

It's a common mistake to have code like:

mod foo;
mod bar {
  mod foo;
}

Many newcomers expect this to have two instances of the same module foo. This in fact declares two separate foo modules which have different types (even though they have the same contents).

We should print a suppressable warning about when this happens. Something along the lines of second inclusion of foo.rs, did you mean to "use" the module instead?

[dcrewi]

This is issue #5765 and I had started to write a patch for it. I suppose I can fix it up.

[alexcrichton]

Ah yes, dupe of #5765