Improve deployment of symlinked webapplications

[gregw]

Target Jetty version(s)
9, 10

Enhancement Description

After #6114 we can now deploy symlinked war files with the correct context path. eg.

   webapps
      + contextA.war -> /var/lib/myapp.war
      + contextB.war -> /var/lib/myapp.war

But the following does not work correctly:

   webapps
      + contextA -> /var/lib/myapp/
      + contextB -> /var/lib/myapp/

[gregw]

I think that at deployment time, either the deployer or the WebAppContext needs to resolve the base resource to its real path, but use the linked path for determining the context path (if none is set).
Removing any symlinks in the base Resource at deployment will simplify the considerations of #6540

[gregw]

It looks like this can be fixed by resolving to the real file either in the deployer or in the ContextHandler. This first corrects this issue only for deployed webapps, whilst the later will resolve for all usages.