These functions are deferring to NamedNodeMap::get_attribute which
already takes a StringView. This changes also leads to finding some
places which were passing though a const char* instead of an entry from
Attribute names. Fix that where applicable, and switch to has_attribute
in some of those places instead of deprecated_attribute where
equivalent.
Ideally this should be taking a 'FlyString const&', but to continue
porting away from DeprecatedString, just leave a FIXME for now.