package: refactor listing of extractor dependencies

package: refactor listing of extractor dependencies

Don't special case $(XZCAT) when constructing DL_TOOLS_DEPENDENCIES. The next
commit will introduce another extractor that automatically builds when not
installed. Introduce EXTRACTOR_DEPENDENCY_PRECHECKED_EXTENSIONS that lists
archive extensions for which the extractor is already checked in
support/dependencies/check-host-foo.mk. Use this in the newly introduced
extractor-dependency to populate DL_TOOLS_DEPENDENCIES.

Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
[Thomas: add missing space after "firstword", as noticed by Thomas DS.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>