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)

[jszumski]

Breaks on paths with spaces and stomps on the current line contents

Suggested change

	print("/usr/bin/sed -i \"\" '1i\\\n\(lines)' \(relativePath)")
	print("/usr/bin/sed -i \"\" '1i\\\n\(lines)\n' '\(relativePath)'")

[jszumski]

Inserting these on line 1 ends up colliding with our copyright preamble:

-//
+import SomeModuleName//
 // Copyright © Square, Inc. All rights reserved.
 //

Maybe we find the line of the first import statement instead? Even better would be to examine the current list and insert it alphabetically into the right place.

[keith]

yea this was just a quick and easy solution. as far as importing them in order it's probably easier to just let swiftlint handle that if you enforce that (it even autocorrects that case)

[jszumski]

When dependentUnit is a .m file the moduleName is the empty string, is that expected?

[keith]

hrm. interesting. i would expect that with -fmodules it might be set, but maybe that's just not at the right level for this. i guess in the meantime worst case is we ignore units with empty module names so we don't try to do anything weird and insert an empty string. i don't think we would be able to trace back to a header, but if you're using modules it seems like it should be possible to trace back to that (but then i would expect the unit to be for the module maybe, not the single .m file, similar to the SDK's modules). I didn't hit this since we don't have any objc