Generate dependencies & destinations with metadata

This commit obviates the need for separate `metadata2dependencies` and
`metadata2destinations` transforms and simply bundles their
functionality into `expandmetadata`.

Making this work right is a bit tricky because we are outputting the
main document to `stdout` but want the other result documents to be
output to `BUILDDIR`. The best solution would be to just read in the
build directory inside of the transform and use it to determine the
output location, but unfortunately `exsl:document` does not support
dynamic computation of the destination directory. The current solution
is instead to `cd` into the build directory in a subshell before
calling `xsltproc`.