Give a better error message when an import isn't found but a namespace of the same name exists

[DanielRosenwasser]

See #10463.

foo.d.ts

declare module foo {
    // ...
}

bar.ts

import foo = require("foo");

"foo" won't be found, but if the compiler can resolve an identifier named foo to a namespace that uses the declare module syntax, we should probably give an error like

Cannot find module 'foo'.
  A legacy-style namespace with the same name was found.
    Try changing the declaration to use quotes (e.g. 'declare module "foo"').

[RyanCavanaugh]

Somewhat skeptical that turning the namespace declaration into an external module declaration is the most likely fix. I feel like people making this error are more likely writing global code and thinking they need to do some incantation to access things across file boundaries

[DanielRosenwasser]

I think as soon as you see an import, you can figure the user is trying to use modules, but I see what you're saying.