{ood,sources,targets}: fix relative paths; turn missing targets into sources.

When we check dependencies and a previously-is_generated dependency
existed before, but no longer does, forget that it was is_generated.
This slightly improves the situation where as a project evolves, a file
that used to be a target gets removed, and then later is re-added as a
static source file.  (It doesn't fix the other variant, where a file is
changed from target to source in a single atomic change, and is never
missing.  That one will be trickier to handle.)

While adding a test for this behaviour, I discovered that redo-sources,
redo-targets, and redo-ood were reporting their output relative to
STARTDIR instead of relative to $PWD, so fix that too.

clean.do

@@ -10,3 +10,4 @@ rm -rf minimal/.do_built minimal/.do_built.dir website
 redo t/clean Documentation/clean version/clean
 rm -f *~ .*~ */*~ */.*~ *.pyc install.wrapper
 find . -name '*.tmp' -exec rm -fv {} \;
+find . -name '*.did' -exec rm -fv {} \;