* interfaces: introduce SnapAppSet and forward one to the security backends
* snap: implement methods on snap.Info for getting apps and hooks for slots and plugs
This will enable us to remove the Hooks fields from the SlotInfo and
PlugInfo structs.
* interfaces: implement methods on SnapAppSet in terms of methods on snap.Info
* snap, interfaces: replace usage of {Plug,Slot}Info.SecurityTags with methods on SnapAppSet
* i/builtin: replace slotAppLabelExpr and plugAppLabelExpr with corresponding methods on SnapAppSet
* snap, o/snapstate, interfaces: remove Hooks field on snap.PlugInfo and snap.SlotInfo
* builtin, interfaces: fix tests that use Specification that now have a SnapAppSet
* snap: add tests for new methods on Info
* interfaces, i/builtin: port over some tests for SnapAppSet methods {Plug,Slot}LabelExpression
* interfaces: test PlugSecurityTags and SlotSecurityTags methods
* interfaces: add doc comments to SnapAppSet and methods
* i/builtin: remove ported over tests
* interfaces, many: require that SnapAppSet methods for getting security tags are called with plug/slot that comes from correct snap
Many tests did not properly adhere to this requirment, so they had to be
modifed to modify this rule.
Additionally, a hack was inroduced in the methods for getting label
expressions on the SnapAppSet. If a plug/slot did not originate from the
same snap that the SnapAppSet was created from, then we will use the
snap.Info that the plug/slot carries in the method instead. This will
fail to work once component hooks are introduced, so this will need to
be resolved by then.
* interfaces: test fallback for using LabelExpr methods with mismatch plug/slot
* snap: correct placement of TODOs to preserve doc comments
* snap: add doc comments for Plug/Slot.Unscoped
* interfaces: test for using SecurityTagsForPlug and SecurityTagsForSlot with wrong snap
* interfaces: tweak error messages in SnapAppSet SecurityTags methods
* i/builtin: fix missed conflict
* i/apparmor: add doc comment to Specification.appSet
* snap: fix doc coment on PlugInfo.Unscoped
