Archive-Extract is a wrapper for many archive formats. Each format
has multiple supported decompressors.
This commit declares the dependencies on decompressors using virtual
(dummy, empty) packages because current RPM does not support
dijunction. Once rich dependencies become supported by RPM, they can
be replaced by simple boolean expression.
Originally, Archive-Extract did not require any decompressor. But it
turned out, that some applications does expect it and it surprises
users as this is in contrast to the documentation of the Perl module.
See bug #1217352 for more details.