PDL: Change the scoping of packet and group declarations

Packet, struct, and group previously lived in different scopes,
which allowed packets and structs to have the same name.
This impacts the rust code generation as packets and structs are
disambiguated by adding appropriate suffixes, which is undesirable.

Packets and groups are now declared in the same namespace
as structs and other typedef declarations. An error is raised
if a packet is declared with the same name as another struct or
group declaration.

Test: pdl_inline_tests, validate hci_packets.pdl file
Bug: 228327522
Change-Id: I8055bed118e04de7297e240ac075ca529b5a8f8a