Add support for adding missing imports

[keith]

This works by collecting all definition USRs from all units, and cross referencing all reference USRs from each file, until there are no unhandled USRs remaining.

[keith]

there are a few cases this adds imports for that might be surprising:

• In this case module c gets an import for module a

// module a
class A {
   let property: String
}

// module b
class B: A {}

// module c
self.b.property

• typealiases + extensions are a bit weird, this case gets an import to A because the extension is technically on A (AFAIUI):

// module a
struct A {}

// module b
typealias B = A

extension B {
  func foo() {}
}

// module c
B().foo()

maybe some more, but most were pretty understandable and more surprising that they could be missing (mostly the usual suspects like extensions on system types, operators, uses of callAsFunction etc)