bitbake: bitbake-user-manual: Rewrite dependency dot file generation

bitbake: bitbake-user-manual: Rewrite dependency dot file generation

The package-depends.dot and pn-depends.dot files are inaccurate,
missing out key dependencies such those made via the [depends]
flags. As such they can be misleading to the user.

They mainly exist for historical reasons, coming from a time
before we had task based execution.

This commit removes the two dated file formats and replaces
them with a recipe-depends.dot which is a flattened version
of task-depends.dot.

The old format files are removed if present so that the user
can't get confused about why data might not match between files.

The code is also rewritten to use 'with f: f.write()' syntax as
is more commonly used now. Also update the docs to match the change.

(Bitbake rev: f82537d27f2a5bf9d576aa841593db9ec0985ea8)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>