Explicitly fail on downgrades in the AIX package provider

Downgrades do nothing because the package is superseded by a newer
version, but still don't exit non-zero. The only way to downgrade is to
force the installation, which requires us to ignore dependencies. As
this isn't palatable, we now explicitly fail if we see that the
package was superseded by a newer version.

This also fixes an issue of using << vs. +=, which would cause the
resource's name to be modified during installation.