package/pkg-utils.mk: rename extractor-dependency to extractor-system-dependency

package/pkg-utils.mk: rename extractor-dependency to extractor-system-dependency

The extractor-dependency macro returns which system-provided tools are
needed to be able to extract the archive passed as argument. The
result of this macro is added to DL_TOOLS_DEPENDENCIES so that the
logic in support/dependencies/ verifies that the necessary tools are
provided by the system.

However, we are going to add another macro, extractor-pkg-dependency,
which says which Buildroot packages are needed to extract the archive
passed as argument. Indeed, for those archive types, if the extractor
is not provided system-wide, we build it as a host Buildroot package.

To clarify the distinction between the upcoming
extractor-pkg-dependency and existing extractor-dependency, we rename
the latter to extractor-system-dependency.

We take this opportunity to extend the documentation of this macro.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>