Fixes#7885#7193 added the patches_to_apply function to collect patches which are then
applied in Package.do_patch. However this only collects patches that are
associated with the Package object and does not include Spec-related patches
(which are applied by dependents, added in #5476).
Spec.patches already collects patches from the package as well as those applied
by dependents, so the Package.patches_to_apply function isn't necessary. All
uses of Package.patches_to_apply are replaced with Package.spec.patches.
This also updates Package.content_hash to require the associated spec to be
concrete: Spec.patches is only set after concretization. Before this PR, it was
possible for Package.content_hash to be valid before concretizing the associated
Spec if all patches were associated with the Package (vs. being applied by
dependents). This behavior was unreliable though so the change is unlikely to
be disruptive.
c97d058ce3