til: introduce "blank" built-in module

rtv special-cased handling a nil module to mean clear the
fragment, and called this "none"

But it really makes more sense for rtv to treat "none" as not
doing anything at all for its snow_module - not even blanking.
And it would be nice to have a consistent way to express a blank
module throughout rototiller, so this introduces a concept of
built-in modules accessible only via explicit lookup by name
which don't get enumerated via til_get_modules(), as they're
inherently uninteresting more utility-oriented modules for use by
other modules.

For now it's only "blank" that constitutes the built-ins list,
but expanding this is only a matter of introducing more modules
there.  Future commits will rework rtv to use "blank" in place of
its current "none", and rtv's "none" will be reworked to
represent no snow mechanism at all, obviating the need to specify
snow_duration=0,snow_module=none required today.

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