(#8596) Detect resource alias conflicts when titles do not match

The introduction of composite namevars caused the resource title used in
resource aliases to be set as an array, even when the resource only had one
namevar. This would fail to conflict with non-alias entries in the resource
table, which used a string for the title, even though the single element array
contained the same string.

Now, we flatten the key used in the resource table, so that single element
arrays are represented as strings, and will properly conflict with resource
titles.

Paired-With: Jacob Helwig <jacob@puppetlabs.com>