to do
/////

* Use test.loader_tests

    + builtin
    + frozen
    + extension
    + source

* Reorganize support code.

    + Separate general support code and importer-specific (e.g. source) support
      code.
        - Create support modules for each subdirectory (as needed).
    + Add a file loader mock that returns monotonically increasing mtime.
        - Use in source/test_reload.
        - Use in source/test_load_module_mixed.

* API simplification?

    + read_source -> get_data/source_path
    + read_bytecode -> get_data/bytecode_path
    + write_bytecode -> complete set of bytes for bytecode instead of
      individual arguments.

* Create meta_path importer for sys.path.

* OPTIMIZE!

  + Write benchmark suite.
  + Fast path common cases.

    - Absolute name from sys.path.
    - Relative name from sys.path.

* Implement PEP 302 protocol for loaders (should just be a matter of testing).

    + Built-in.
    + Frozen.
    + Extension.
    + Source/bytecode.

* Public API to expose (w/ docs!)

  + abc

      - Finder

        * find_module

      - Loader

        * load_module

      - ResourceLoader(Loader)

        * get_data

      - InspectLoader(Loader)

        * is_package
        * get_code
        * get_source

      - (?) SourceLoader(ResourceLoader)

        * source_path
        * bytecode_path
        * write_bytecode

  + util

      - get_module decorator (new name)
      - check_name decorator (new name)
      - resolve_name

  + machinery

      - (?) Chained path hook/finder
      - BuiltinImporter
      - FrozenImporter
      - (?) FileFinder
      - Extensions importers

          * ExtensionFinder
          * (?) Loader

      - Source/bytecode importers

          * SourceFinder
          * (?) Loader

* Bootstrap importlib as implementation of builtins.__import__