Missing /usr/include/ headers on macOS Catalina

I just installed the macOS Catalina 10.15 GM. Unfortunately none of my app frameworks compile. The system header files were not found. On macOS Mojave there was a workaround, but it no longer works, the file won't be dowloaded (the workaround is explained here)

When typing

xcrun --show-sdk-path

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk

This is how my module.modulemap looks like:

module PrivateNetwork [system] { header "/usr/include/sys/socketvar.h" header "/usr/include/net/if_dl.h" header "/usr/include/net/if_types.h" header "/usr/include/net/if.h" header "/usr/include/netinet/in.h" header "/usr/include/netinet/tcp.h"  header "/usr/include/netinet/tcp_var.h" header "/usr/include/netinet/tcpip.h" header "/usr/include/netinet/tcp_fsm.h" header "/usr/include/netinet/ip.h" header "/usr/include/netinet/ip6.h"  export * }

Error:

Header '/usr/include/sys/socketvar.h' not found