A bit of a nitpick - LiteX still mostly needs Vivado installed for now for 7 series FPGAs. There's a project that's very far along in reverse engineering the Xilinx bitstream (https://github.com/f4pga/prjxray), but it's still missing many features (PCIe, SERDES, hardware multipliers, etc.). You can run on the open source toolchain but you lose so many of the reasons to actually use a Xilinx FPGA.
The only fully supported reverse engineered FPGAs at the moment are the Lattice ECP5(-5G), iCE40, and QuickLogic EOS families.