AAPT2: Compile --zip flag

Added a --zip flag similar to --dir that allows resources to be passed
into "aapt2 compile" using a zip file.

Also refactored Compile.cpp to be easier to mock and test in the future.

Bug: 74574557
Test: aapt2_tests
Change-Id: Idb90cb97e23a219525bdead38220cbf7bc6f3cab
diff --git a/tools/aapt2/io/FileSystem.h b/tools/aapt2/io/FileSystem.h
index 6be8807..fb6bf6e 100644
--- a/tools/aapt2/io/FileSystem.h
+++ b/tools/aapt2/io/FileSystem.h
@@ -59,6 +59,10 @@
  public:
   FileCollection() = default;
 
+  /** Creates a file collection containing all files contained in the specified root directory. */
+  static std::unique_ptr<FileCollection> Create(const android::StringPiece& path,
+                                                std::string* outError);
+
   // Adds a file located at path. Returns the IFile representation of that file.
   IFile* InsertFile(const android::StringPiece& path);
   IFile* FindFile(const android::StringPiece& path) override;