drop httpclient dependency, which wasn't actually being used by Ivy
account for localOnly when cache subclass overrides isChanging
Fix for cache issues when developing sbt itself.

Ivy still seems to remember the resolver that found the metadata
and records that as the resolver to use for the artifact.  Previous
fixes have attempted to ignore or blank this resolver.  The issue
this fix addresses comes from inter-project not being ignored/blanked.
inter-project can't provide artifacts because it is only for projects
in the current build under development and not for modules in a repository.
The symptom is that compiling a build definition fails because the
sbt jars aren't on the classpath, but it is ultimately due to the
inter-project resolver incorrectly being used to resolve artifacts.