This is xnu-8019. See this file in:
module Darwin.POSIX.netinet [system] {
  module in {
    header "netinet/in.h"
    export *

    #ifdef XNU_KERNEL_PRIVATE
    // netinet6/in6.h is conceptually a part of netinet/in.h, and so
    // belongs to this module, but needs to be excluded or else clang
    // will attempt to compile in6.h independently. And that fails
    // because in6.h can only be included from in.h, it cannot be used
    // in any other context.
    #endif
    exclude header "netinet6/in6.h"
  }

  module tcp {
    header "netinet/tcp.h"
    export *
  }
}

module Darwin.netinet [system] {
  #ifdef XNU_KERNEL_PRIVATE
  // The PLATFORM_MacOSX headers are in the public SDK for macOS,
  // but mastered out of the public SDK for the other OSes.
  #endif
  #ifdef PLATFORM_MacOSX
  module bootp {
    header "netinet/bootp.h"
    export *
  }

  #endif
  module icmp6 {
    header "netinet/icmp6.h"
    export *
  }

  #ifdef PLATFORM_MacOSX
  module icmp_var {
    header "netinet/icmp_var.h"
    export *
  }

  module if_ether {
    header "netinet/if_ether.h"
    export *
  }

  module igmp {
    header "netinet/igmp.h"
    export *
  }

  module igmp_var {
    header "netinet/igmp_var.h"
    export *
  }

  #endif
  module in_pcb {
    header "netinet/in_pcb.h"
    export *
  }

  module in_systm {
    header "netinet/in_systm.h"
    export *
  }

  #ifdef PLATFORM_MacOSX
  module in_var {
    header "netinet/in_var.h"
    export *
  }

  #endif
  module ip {
    header "netinet/ip.h"
    export *
  }

  module ip6 {
    header "netinet/ip6.h"
    export *
  }

  module ip_icmp {
    header "netinet/ip_icmp.h"
    export *
  }

  #ifdef PLATFORM_MacOSX
  module ip_var {
    header "netinet/ip_var.h"
    export *
  }

  module tcp_fsm {
    header "netinet/tcp_fsm.h"
    export *
  }

  module tcp_seq {
    header "netinet/tcp_seq.h"
    export *
  }

  #endif
  module tcp_timer {
    header "netinet/tcp_timer.h"
    export *
  }

  module tcp_var {
    header "netinet/tcp_var.h"
    export *
  }

  #ifdef PLATFORM_MacOSX
  module tcpip {
    header "netinet/tcpip.h"
    export *
  }

  #endif
  module udp {
    header "netinet/udp.h"
    export *
  }
  #ifdef PLATFORM_MacOSX

  module udp_var {
    header "netinet/udp_var.h"
    export *
  }
  #endif
}