The `#%module-begin' of `racket/base' and `scheme/base' now introduces
a suitable `configure-runtime' submodule, instead of using the
`module->language-info' path.
A submodule is a lot easier to work with, as illustrated by the
removal of the `racket/private/lang' and `scheme/private/lang'
languages.
Also, add `#%printing-module-begin', which is the old `#%module-begin'
(i.e., the one that doesn't introduce a `configure-runtime' submodule).
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