(PUP-2182) Add virtual_packages feature to package type.

[peterhuene]

In 3.5.x, the yum package provider changed to accept virtual package
names (packages that provide other package names).

That change in behavior caused some undesirable side-effects. As we did
not give users fair warning that this behavior was changing, we have
decided to change the yum provider back to only accepting "real" package
names by default.

To keep the support for virtual package names, a "virtual_packages"
feature has been implemented on the package type and supported on the
yum package provider. The feature adds the "allow_virtual" parameter
that controls whether or not virtual package names will be allowed for
install/uninstall.

For now, the default value for this parameter will be false to restore
the previous behavior. However, a deprecation warning will be issued to
let users know that the default will eventually be changed to true.
Users can prevent the deprecation warning by explicitly setting the
parameter to either true or false.

[peterhuene]

Note: this has already been merged into master. I accidentally targeted it at master since I thought it was a 3.7.x fix, but it's intended for 3.6.1. After we merge this, I'll resolve the minor conflict when merging stable to master.

[puppetcla]

CLA signed by all contributors.