allow underscore in package names

The current Tumbleweed repository has more than three thousand packages with an underscore in their package name. We drag this patch around since a decade. Please review and consider merging it.

--- a/lib/cudf_type_lexer.mll
+++ b/lib/cudf_type_lexer.mll
@@ -24,7 +24,7 @@
 let blank = [ ' ' '\t' ]
 let blanks = blank+
 let ident = lower_letter (lower_letter | digit | '-')*
-let pkgname = (letter | digit | ['-' '+' '.' '/' '@' '(' ')' '%'])+
+let pkgname = (letter | digit | ['-' '+' '.' '/' '@' '(' ')' '%' '_'])+
 
 rule token_cudf = parse
   | "true!"            { VPKGTRUE }