Bug in gb_Package_use_unpacked?

[Stephan Bergmann]

With a configuration containing --enable-ext-numbertext, my latest 
not-from-scratch build after 
<https://git.libreoffice.org/core/+/e6165b7cac5d91458d61da3de35486cde3004897%5E%21> 
"tdf#136368 bump to libnumbertext 1.0.7" failed with

> ~/lo/core/external/libnumbertext/ExternalPackage_numbertext.mk:12: *** file ~/lo/core/workdir/UnpackedTarball/libnumbertext/data/ga.sor does not exist in the tarball.  Stop.

(and `make UnpackedTarball_libnumbertext.clean` worked around the issue).

I think the issue is that

> commit df4ff3547078c17cf69f7574dd7995cc51cc7934
> Author: David Tardon <dtardon at redhat.com>
> Date:   Mon Aug 20 11:15:05 2012 +0200
> 
>     add support for external tarballs to gbuild
>     
>     Change-Id: Ic6645e9943b2445ebb37bb99114f777527c69af9
[...]
> diff --git a/solenv/gbuild/Package.mk b/solenv/gbuild/Package.mk
> index e1d199e4f142..23aeceeb8fe0 100644
> --- a/solenv/gbuild/Package.mk
> +++ b/solenv/gbuild/Package.mk
> @@ -114,4 +114,10 @@ $(foreach file,$(3),$(call gb_Package_add_file,$(1),$(2)/$(file),$(file)))
>  
>  endef
>  
> +# Package files from unpacked tarball of an external project
> +define gb_Package_use_unpacked
> +$(call gb_Package_get_preparation_target,$(1)) :| $(call gb_UnpackedTarball_get_target,$(2))
> +
> +endef
> +
>  # vim: set noet sw=4:

made the dependency on gb_UnpackedTarball_get_target,libnumberext 
(triggered via `$(eval $(call 
gb_ExternalPackage_ExternalPackage,libnumbertext_numbertext,libnumbertext))` 
-> gb_ExternalPackage_ExternalPackage_internal -> 
gb_Package_use_unpacked) order-only.  The old unpacked 
libnumbertext-1.0.6.tar.xz already exists at 
workdir/UnpackedTarball/libnumbertext, but lacks data/ga.sor which the 
new external/libnumbertext/ExternalPackage_numbertext.mk now wants to 
package.