…er: update snap-confine to be able to handle security tags that come from component hooks

An example of a security tag from a component hook would be:
"snap.name+comp.hook.install"

And one with an instance key:
"snap.name_instance+comp.hook.install"

Something important to note is how these are encoded as udev tags.
Currently, when converting a security tag to a udev tag, we replace all
'.' characters in the tag with '_' characters because systemd limits
udev tags to having only alphanumeric characters, with the addition of
the characters '-' and '_'. Since security tags can now contain '+'
characters, those will be encoded as two consecutive '_' characters.

For example:
"snap.name+comp.hook.install" -> "snap_name__comp_hook_install"
"snap.name_instance+comp.hook.install" -> "snap_name_instance__comp_hook_install"

This allows the conversion to maintain its reversibility.

… + as __

…all matches

…to validate_as_snap_or_component_name

…to take in error details

…_validate

Make sure it fails to validate component names with instance keys, test
that we fail to validate a component name that matches against a wrong
instance key.

…t tests

…uct field

… array size

… clarity