checker: remove mod_file_cacher exception

vlib/v/builder/builder.v

@@ -197,7 +197,8 @@ fn module_path(mod string) string {
 
 pub fn (b Builder) find_module_path(mod, fpath string) ?string {
 	// support @VROOT/v.mod relative paths:
-	vmod_file_location := vmod.mod_file_cacher.get_by_file(fpath)
+	mcache := vmod.get_cache()
+	vmod_file_location := mcache.get_by_file(fpath)
 	mod_path := module_path(mod)
 	mut module_lookup_paths := []string{}
 	if vmod_file_location.vmod_file.len != 0 && vmod_file_location.vmod_folder !in b.module_search_paths {

vlib/v/checker/checker.v

@@ -583,10 +583,7 @@ fn (mut c Checker) fail_if_immutable(expr ast.Expr) {
 					c.error('`$it.name` is immutable, declare it with `mut` to make it mutable',
 						it.pos)
 				}
-			} else if it.name in c.const_names {
-				if it.name .contains('mod_file_cacher') {
-					return
-				}
+			} else if it.name in c.const_names {          
 				c.error('cannot modify constant `$it.name`', it.pos)
 			}
 		}

vlib/v/parser/comptime.v

@@ -15,7 +15,8 @@ const (
 )
 
 fn (mut p Parser)resolve_vroot(flag string) string {
-	vmod_file_location := vmod.mod_file_cacher.get_by_folder(p.file_name_dir)
+	mcache := vmod.get_cache()
+	vmod_file_location := mcache.get_by_folder(p.file_name_dir)
 	if vmod_file_location.vmod_file.len == 0 {
 		// There was no actual v.mod file found.
 		p.error('To use @VROOT, you need' + ' to have a "v.mod" file in ${p.file_name_dir},' +

vlib/v/scanner/scanner.v

@@ -807,7 +807,8 @@ pub fn (mut s Scanner) scan() token.Token {
 			}
 			if name == 'VMOD_FILE' {
 				if s.vmod_file_content.len == 0 {
-					vmod_file_location := vmod.mod_file_cacher.get_by_file( s.file_path )
+					mcache := vmod.get_cache()
+					vmod_file_location := mcache.get_by_file( s.file_path )
 					if vmod_file_location.vmod_file.len == 0 {
 						s.error('@VMOD_FILE can be used only in projects, that have v.mod file')
 					}

vlib/v/vmod/vmod.v

@@ -33,6 +33,7 @@ pub:
 	vmod_folder string
 }
 
+[ref_only]
 pub struct ModFileCacher {
 mut:
 	cache map[string]ModFileAndFolder
@@ -155,6 +156,8 @@ fn (mut mcache ModFileCacher) get_files(cfolder string) []string {
 	return files
 }
 
-pub const (
-	mod_file_cacher = new_mod_file_cacher() // used during lookup for v.mod to support @VROOT
-)
+// used during lookup for v.mod to support @VROOT
+const ( private_file_cacher = new_mod_file_cacher() )
+pub fn get_cache() &ModFileCacher {
+	return private_file_cacher
+}