Make lexer-helpers.hh internal to fix a clang-tidy error

(cherry picked from commit 8be24f58f2)

src/libexpr/include/nix/expr/meson.build

@@ -20,7 +20,6 @@ headers = [config_pub_h] + files(
   'gc-small-vector.hh',
   'get-drvs.hh',
   'json-to-value.hh',
-  # internal: 'lexer-helpers.hh',
   'nixexpr.hh',
   'parser-state.hh',
   'primops.hh',

src/libexpr/lexer-helpers.cc

@@ -1,7 +1,4 @@
-#include "lexer-tab.hh"
-#include "parser-tab.hh"
-
-#include "nix/expr/lexer-helpers.hh"
+#include "lexer-helpers.hh"
 
 void nix::lexer::internal::initLoc(YYLTYPE * loc)
 {

src/libexpr/lexer.l

@@ -25,8 +25,7 @@
 #endif
 
 #include "nix/expr/nixexpr.hh"
-#include "parser-tab.hh"
-#include "nix/expr/lexer-helpers.hh"
+#include "lexer-helpers.hh"
 
 namespace nix {
   struct LexerState;