depends: Move source stamps to source dir and misc cleanups

Since the last commit will force rebuilds of all depends, take the opportunity
to clean up a few other things that would trigger rebuilds as well.

- Move source stamps to the sources dir so that SOURCES_PATH is respected for
  "make download".
- Only print "fetching..." when actually downloading a file.
- Avoid using non-deterministic paths for the recipe hash (patch location).
  This should ensure that all builders get the same resulting build-ids.
- Use a per-package source paths. This will allow for removing old source files
  in the future.
- Use a host-agnostic path for downloads which gets cleaned up properly.

depends/Makefile

@@ -31,6 +31,7 @@ endif
 
 base_build_dir=$(BASEDIR)/work/build
 base_staging_dir=$(BASEDIR)/work/staging
+base_download_dir=$(BASEDIR)/work/download
 canonical_host:=$(shell ./config.sub $(HOST))
 build:=$(shell ./config.sub $(BUILD))