• with only one source bundle (per JEP-296), we can use src instead of
  srcs, and avoid the need to cd in prePatch

• fetch sources from jdk10u instead of jdk10, to make it easier to
  grab updates when they start coming.

• removed commented-out code that became irrelevant in the 8 -> 9
  transition (*.pf files, infinality font rendering)

• create jdk10, jre10, and jre10_headless attributes in
  all-packages.nix

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.