use standard Maven caching for local repository site descriptors

I've found that the site plugin (or perhaps Doxia) is caching 404's for site descriptors in the local repository as empty files, as a form of added efficiency for descriptors that are often not present.

However, this has a downside - I had one cached due to switching to a different parent before it could be downloaded - so even when it was available it silently used an empty parent descriptor (even through -U), causing the Maven site to be deployed with no skin.

Likewise, when the 17-SNAPSHOT maven-parent was cleaned up, the previously working site build would no longer have a valid parent descriptor and silently failed.

There should be a clear indication in the build that no site descriptor was found (or not used due to previously caching), and where possible rely on Maven's built in handling for caching resolution failures.