v.builder: improve builder error messages (fix #14386) (#14421)

vlib/v/builder/builder.v

@@ -556,7 +556,9 @@ pub fn (mut b Builder) print_warnings_and_errors() {
 				}
 			}
 			if redefines.len > 0 {
-				eprintln('redefinition of function `$fn_name`')
+				ferror := util.formatted_error('builder error:', 'redefinition of function `$fn_name`',
+					'', token.Pos{})
+				eprintln(ferror)
 				for redefine in redefines {
 					eprintln(util.formatted_error('conflicting declaration:', redefine.fheader,
 						redefine.fpath, redefine.f.pos))

vlib/v/checker/tests/fn_duplicate.out

@@ -1,13 +1,12 @@
-redefinition of function `main.f`
+builder error: redefinition of function `f`
 vlib/v/checker/tests/fn_duplicate.vv:1:1: conflicting declaration: fn f()
     1 | fn f() {
       | ~~~~~~
-    2 |     
-    3 | }
-vlib/v/checker/tests/fn_duplicate.vv:5:1: conflicting declaration: fn f(i int)
-    3 | }
-    4 | 
-    5 | fn f(i int) {
+    2 | }
+    3 |
+vlib/v/checker/tests/fn_duplicate.vv:4:1: conflicting declaration: fn f(i int)
+    2 | }
+    3 |
+    4 | fn f(i int) {
       | ~~~~~~~~~~~
-    6 |     
-    7 | }
+    5 | }

vlib/v/checker/tests/fn_duplicate.vv

@@ -1,7 +1,5 @@
 fn f() {
-	
 }
 
 fn f(i int) {
-	
 }

vlib/v/util/errors.v

@@ -86,14 +86,18 @@ pub fn formatted_error(kind string, omsg string, filepath string, pos token.Pos)
 			path = path.replace_once(util.normalised_workdir, '')
 		}
 	}
-	//
-	position := '$path:${pos.line_nr + 1}:${mu.max(1, pos.col + 1)}:'
+
+	position := if filepath.len > 0 {
+		'$path:${pos.line_nr + 1}:${mu.max(1, pos.col + 1)}:'
+	} else {
+		''
+	}
 	scontext := source_file_context(kind, filepath, pos).join('\n')
 	final_position := bold(position)
 	final_kind := bold(color(kind, kind))
 	final_msg := emsg
 	final_context := if scontext.len > 0 { '\n$scontext' } else { '' }
-	//
+
 	return '$final_position $final_kind $final_msg$final_context'.trim_space()
 }