Don't check package.installed in _mark_concrete if value=True (#5634)

* spec and spec.package.spec can refer to different objects in the
database. When these two instances of spec differ in terms of
the value of the 'concrete' property, Spec._mark_concrete can
fail when checking Spec.package.installed (which requires
package.spec to be concrete). This skips the check for
spec.package.installed when _mark_concrete is called with
'True' (in other words, when the database is marking all specs
as being concrete).

* add test to confirm this fixes #5293