DependenciesScanner: implement protocol for batch module dependencies scan

[nkcsgexi]

This scanning mode allows swift-driver to query module dependencies in a batch
and in a more granular way. In short term, it could help solve a problem that
clang module dependencies may vary if target triple changes. In a longer term,
we could break a holistic dependencies graph into smaller pieces for better caching
and reusing.

This change doesn't include the implementation of using the specified scanner
arguments to set up Clang dependencies scanner. It will come in later commits.

[nkcsgexi]

@swift-ci please smoke test

[artemcm]

Is the only reason we include a file extension here to distinguish whether or not that's a Swift or Clang module? It seems that for either scanner we then strip it away. The file extension is kind of a downstream implementation detail here; sure, once we build them they end up being a .swiftmodule or a .pcm, but at the scanning stage we have neither and all we have to search by is a module name.

Do you think it would be cleaner to have the JSON key indicate what kind of module it is and leave module name value just have the name the scanners can use directly?

[nkcsgexi]

Yeah, I think adding a more explicit key is totally reasonable. Should we add a boolean value key isSwiftModule?

[artemcm]

Yeah, either an additional boolean value, or have the module name key itself be either clangModuleName or swiftModuleName, kinda like this:

enum BatchScanModuleInfo {
  case swift(SwiftScanModuleInfo)
  case clang(ClangScanModuleInfo)
}
struct ClangScanModuleInfo: Codable {
  var clangModuleName: String
  var pcmArgs: [String]
  var outputPath: String
}
struct SwiftScanModuleInfo: Codable {
  var swiftModuleName: String
  var outputPath: String
}

Whichever makes more sense to deal with in the scanner.

[nkcsgexi]

ah, That's a great idea. Will differentiate the key for a concise representation.

[nkcsgexi]

@swift-ci please smoke test

[nkcsgexi]

@swift-ci Please smoke test OS X platform

[nkcsgexi]

@swift-ci Please smoke test OS X platform

[nkcsgexi]

@swift-ci Please smoke test OS X platform