{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.

t/351-deps-forget/.gitignore

@@ -0,0 +1,6 @@
+bork
+bork.do
+bork.log
+sub
+sub.log
+targets.out