Move platform-specific pkgdb check to ebuild.py

In preparation for adding similar pre-flight checks to the yumpkg
providers, this commit moves the pre-flight checks done for Gentoo to
ebuild.py.
Use new pkg.checkdb function for pre-flight checks

This establishes a new convention for pkg.installed states: If it is
considered necessary, pre-flight checks can be done to determine if
possible matches can be found for packages with ambiguous names.
Currently this applies to Gentoo-based distros for which package names
are provided without the category, and will (in a later commit) be
expanded to support the yumpkg providers.

In the case of yumpkg{,5}, yum will automagically resolve package names
like "vim" to their actual name, "vim-enhanced", but this breaks
post-install verification of package targets since "vim" !=
"vim-enhanced". The solution, like with Gentoo, will be to attempt to
detect the possible matches for a given package name, and then fail out
of the state. This way, the user will be given a clear indication of the
proper changes to make to their states, and ambiguous package names will
not be tolerated.
Add kwargs to pre-flight check
yumpkg5: add pkg suggestions for nonexistant packages

See issue #6298 for more information.
yumpkg: add pkg suggestions for nonexistant packages

See issue #6298 for more information.
Merge pull request #7117 from terminalmage/issue6298

yumpkg{,5}: add pkg suggestions for nonexistant packages