Spec.satisfies accesses Spec.concrete as property (#2928)

* Spec.satisfies accesses Spec.concrete as property

Fixes #2760

When copying a spec, _concrete is always set to False for each
dependency. "Spec.satisfies" was accessing the member "_concrete"
directly instead of using the property "concrete". This means that
if you copy a spec, the dependencies will be considered equal, but
did not necessarily satisfy one another. Spec.satisfies is a
prerequisite for a package to be considered an extension; as a
consequence, an extension with run-time dependencies that were also
extensions did not activate those extensions. This updates
Spec.satisfies to avoid checking the cached member "_concrete"
directly.

* Added test to check for activation of dependency extension

* Added test to check for transitive satisfiability between a spec and its copy