Handle self-destruct packages
These are sometimes useful in situations where a package is obsoleted,
but doesn't have a simple replacement.
The parser can't currently handle "libsolv-self-destruct-pkg()" (the
magic provide name which triggers this behaviour in libsolv) appearing
as a provide, because (i) brackets aren't a valid character in a package
name, and (ii) empty braces isn't permitted for a versioncriteria
following a package name.
For backwards-compatibility (to allow old versions of setup to parse a
setup.ini containing this instruction) and terseness, accept
'_self-destruct' as equivalent to that magic package name.
Requires libsolv >= 0.7.8