Transform absolute links in ResourceTransformers

Prior to this change, ResourceTransformers that transformed resources by
updating the links to other resources, worked only if links were
relative to the resource being transformed.
For example, when the CssLinkResourceTransformer rewrote links within
a "main.css" resource, only links such as "../css/other.css" were
rewritten.

Using relative links is a recommended approach, because it's totally
independent from the application servlet path, context path, mappings...

This change allows absolute links to be rewritten by those Transformers,
provided those links are accurate and point to existing resources.

Issue: SPR-12137