[1/n] Add support for reading JSON project descriptions

[andrewchang-bird]

⚠️ Stack 1 of n ⚠️

Custom build systems such as Buck or Bazel need a way to generate mocks without an .xcodeproj file. This adds support for passing a JSON project description containing all of the required module and dependency graph information usually parsed from an Xcode project. The description format is roughly compatible with the output from $ swift package describe --type JSON, although adding the dependency field needs to be upstreamed.

{
  "targets": [
    {
      "name": "MyLibrary",
      "type": "library",
      "path": "/path/to/my-library",
      "dependencies": [],
      "sources": [
        "SourceFileA.swift",
        "SourceFileB.swift"
      ]
    },
    {
      "name": "MyOtherLibrary",
      "type": "library",
      "path": "/path/to/my-other-library",
      "dependencies": [
        "MyLibrary"
      ],
      "sources": [
        "SourceFileA.swift",
        "SourceFileB.swift"
      ]
    },
    {
      "name": "MyLibraryTests",
      "type": "test",
      "path": "/path/to/my-library-tests",
      "dependencies": [
        "MyLibrary"
      ],
      "sources": [
        "SourceFileA.swift",
        "SourceFileB.swift"
      ]
    }
  ]
}

The --project generator option now accepts arbitrary files. Iff a file with an xcodeproj extension is provided then it is read as an Xcode project. Cache metadata is stored in the same enclosing directory as the project description file under the .mockingbird subdirectory.

Since builds are run outside of an Xcode testing context, it's necessary to optionally provide a host test target with the --testbundle option to enable thunk pruning support.

mockingbird generate \
  --target MyLibrary \
  --testbundle MyLibraryTests \
  --project /path/to/project-desc.json