ResourceLoader: Add support for packageFiles

ResourceLoader: Add support for packageFiles

Package files are files that are part of a module, but are not
immediately executed when the module executes. Instead, they are
lazy-excecuted when require() is called on them. Package files can be
scripts (JS) or data (JSON), and can be real files on the file system,
or virtual files generated by a callback.

Using virtual data files, server-side data and config variables can be
bundled with a module. Support for file-based require() allows us to
import npm modules into ResourceLoader more easily.

The require function passed to each script execution context, which was
previously a reference to the global mw.loader.require() function, is
changed to one that is scoped to the module and the file being executed.
This is needed to support relative paths: require( '../foo.js' ) can
mean a different file depending on the path of the calling file.

The results of require()ing each file (i.e. the value of module.exports
after executing it) are stored, and calling require() on the same file a
second time won't execute it again, but will return the stored value.

Miscellaneous changes:
- Add XmlJsCode::encodeObject(), which combines an associative array of
  XmlJsCode objects into one larger XmlJsCode object. This is needed for
  encoding the packageFiles parameter in mw.loader.implement() calls.

Bug: T133462
Change-Id: I78cc86e626de0720397718cd2bed8ed279579112