This change tracks the set of imported modules in the ast.Module
structure. Although we can in principle gather up all imports simply
by looking through the fully qualified names, that's slightly hokey;
and furthermore, to properly initialize all modules, we need to know
in which order to do it (in case there are dependencies). I briefly
considered leaving it up to MetaMu compilers to inject the module
initialization calls explicitly -- for infinite flexibility and perhaps
greater compatibility with the source languages -- however, I'd much
prefer that all Mu code use a consistent module initialization story.
Therefore, MetaMus declare the module imports, in order, and we will
evaluate the initializers accordingly.
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