* Removed the global state of the resolution cache

* the cache at the package level is only used for static entries (json schema, swagger schema)
* all subsequent utilizations of this cache are taken from a shallow clone of this initial cache
  which will therefore not mutate

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>

* Added PathLoader to ExpandOptions

This allows to bypass package level settings.

In particular, go-openapi/loads can be changed to remove the subtle race conditions
when altering PathLoader at the package level from the Spec() function.

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>

* * adapted package tests to avoid switching the package level variable
* ran tests with go-openapi/validate and go-openapi/loads

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>