ast/compiler: retain unprocessed parsed modules

[srenatus]

It would sometimes be handy -- when using OPA via Go -- to be able to retrieve the modules that the compiler has worked on in pristine state. So far, the code flow goes like this:

1. c.Modules is a map[string]*Module of parsed ast.Module structs, as per the ast package's ParseModule(string, string) (*Module, error)
2. c.Compile() is run
3. c.Modules now contains the modules that have been rewritten by the various compiler stages

The enhancement here would be to add a new compiler options, e.g.

func (c *Compiler) WithKeepModules(yes bool) *Compiler {
	c.keepModules = yes
	return c
}

and if it was provided during compiler setup, we'd copy the modules into c.ParsedModules, a map[string]*Module to add to the compiler struct.

[tsandall]

In addition, the ParsedModules map should include modules pulled in via the module loader callback.