Enable two‐stage builds using a “data” directory

One pattern for building sites is to use a large number of data files
which are all compiled into a single large file, which is then used to
derive the various pages of the website (using expanded archive
functionality). While this can be done using ordinary includes, it is
expensive to do so, because inclusions and transformations on the
“single large file” will re‐happen every time that it is included. What
is actually desired in this case is an intermediate data file, which
has already had all transformations applied, which can then be included
into other files.

This commit establishes a pattern for doing so, where·by certain files
(conventionally, by file extension; by default, `.rdf`) can be
designated as “data files” and built during an initial pass using
includes from a “data directory”, and then themselves be available as
includes for the second stage. Installing installs files from both
stages. Other commands (such as `listout`) currently ignore data files
because they don’t follow common assumptions (they are not built to
`$(BUILDDIR)/stage2/public`, for example); variants or flags to enable
listing these files may be fruitful future work.