[BuildStream] Fetch and extract source code for build target

[Daniel Drake <drake endlessm com>]

Hi,

Thanks for developing buildstream! I'm using it for the first time.

My request may be a little unusual. I would like to download and
extract all the source code for the org.gnome.Platform flatpak
runtime, which is built using buildstream from
https://gitlab.gnome.org/GNOME/gnome-build-meta

At that point I will run some source analysis tool. I don't actually
need to build it.

I've experimented and the closest I've found is:
$ bst track -d all flatpak/platform.bst
$ bst fetch flatpak/platform.bst

Now in ~/.cache/buildstream/sources I have all the tar files, zip
files and git repos; I could script something to unpack them all into
a location of my choosing.

Are there any better approaches here?

If possible, I would also like to exclude build dependencies and just
work with the source code that corresponds to packages that actually
get shipped. For example the above commands download "bison" which is
undoubtedly needed during the build process, but is not shipped in the
final flatpak runtime, so ideally I would like to exclude it from this
analysis.

Thanks
Daniel