Allow multiple source directories

⛩️📰 书社 might be called from another script or make·file, which
might have built files of its own. It would be a pain if each parent
script needed to copy all the source files into a new build directory
at every step, and it’s much easier to just allow ⛩️📰 书社 to support
multiple source directories (one for the original sources, and
additional ones for any files built by other scripts).

Naturally, ⛩️📰 书社 can’t support the same file subpath across
multiple source directories, as these would compile to the same place.
This commit tries to migitage this by just taking the first match, but
it hasn’t been tested and the behaviour should formally be considered
undefined.