Manual cleanup of P4C namespace usage after the introduction of `::P4`

[qobilidop]

This is a followup to https://github.com/p4lang/p4c/issues/4707#issuecomment-2290862470. After https://github.com/p4lang/p4c/pull/4825, we need another round of manual examination of each subdirectory to improve the namespace usages regarding coding style and readability. This issue is for tracking progress on that.

Progress

• backends
  • [ ] bmv2
  • [ ] common
  • [ ] dpdk
  • [ ] ebpf
  • [ ] graphs
  • [ ] p4fmt
  • [ ] p4test
  • [x] p4tools (@fruffy)
  • https://github.com/p4lang/p4c/pull/4930
  • [ ] tc
  • [ ] ubpf
• [ ] control-plane
• [ ] frontends
• [ ] ir
• [ ] lib
• [ ] midend
• [ ] test