Add observability hooks for required modules

[rdw-software]

The basic idea is that a list of dependencies can be generated by observing the virtual file names passed while running test suites etc.

Quick prototype:

local require = _G.require
	_G.require = function(virtualFileName)
		printf("Required module: %s", virtualFileName)
		return require(virtualFileName)
	end

It might make sense to add an event for this so that it can simply be triggered with the trace command? (requires #55 )

[rdw-software]

Will this conflict with LUAZIP searchers?

If vfs.require is used to load files from zip apps, it should also support observability hooks, with no code changes.