Revert "Don't parse "x:x" as a URI"

This reverts commit f90f660b24.

This broke Hydra's release.nix, which contained

  preCheck = ''export LOGNAME=${LOGNAME:-foo}'';

src/libexpr/lexer.l

@@ -90,7 +90,7 @@ FLOAT       (([1-9][0-9]*\.[0-9]*)|(0?\.[0-9]+))([Ee][+-]?[0-9]+)?
 PATH        [a-zA-Z0-9\.\_\-\+]*(\/[a-zA-Z0-9\.\_\-\+]+)+\/?
 HPATH       \~(\/[a-zA-Z0-9\.\_\-\+]+)+\/?
 SPATH       \<[a-zA-Z0-9\.\_\-\+]+(\/[a-zA-Z0-9\.\_\-\+]+)*\>
-URI         [a-zA-Z][a-zA-Z0-9\+\-\.]*\:\/\/[a-zA-Z0-9\%\/\?\:\@\&\=\+\$\,\-\_\.\!\~\*\']+|channel\:[a-zA-Z0-9\%\/\?\:\@\&\=\+\$\,\-\_\.\!\~\*\']+
+URI         [a-zA-Z][a-zA-Z0-9\+\-\.]*\:[a-zA-Z0-9\%\/\?\:\@\&\=\+\$\,\-\_\.\!\~\*\']+
 
 
 %%

tests/lang/lib.nix

@@ -49,7 +49,7 @@ rec {
     if comp (head list2) (head list1) then [(head list2)] ++ mergeLists comp list1 (tail list2) else
     [(head list1)] ++ mergeLists comp (tail list1) list2;
 
-  id = x:x; # sic
+  id = x: x;
 
   const = x: y: x;
 

tests/lang/parse-okay-url.nix

@@ -1,8 +1,7 @@
-[
+[ x:x
   https://svn.cs.uu.nl:12443/repos/trace/trunk
   http://www2.mplayerhq.hu/MPlayer/releases/fonts/font-arial-iso-8859-1.tar.bz2
   http://losser.st-lab.cs.uu.nl/~armijn/.nix/gcc-3.3.4-static-nix.tar.gz
   http://fpdownload.macromedia.com/get/shockwave/flash/english/linux/7.0r25/install_flash_player_7_linux.tar.gz
   ftp://ftp.gtk.org/pub/gtk/v1.2/gtk+-1.2.10.tar.gz
-  channel:nixos-17.09
 ]