checker: make the type C.X is private re-declaration error more informative

vlib/v/checker/checker.v

@@ -3741,8 +3741,9 @@ fn (mut c Checker) ensure_type_exists(typ ast.Type, pos token.Pos) ? {
 		return
 	}
 	sym := c.table.sym(typ)
-	if !c.is_builtin_mod && sym.kind == .struct_ && sym.mod != c.mod && !sym.is_pub {
-		c.error('type `$sym.name` is private', pos)
+	if !c.is_builtin_mod && sym.kind == .struct_ && !sym.is_pub && sym.mod != c.mod {
+		c.error('struct `$sym.name` was declared as private to module `$sym.mod`, so it can not be used inside module `$c.mod`',
+			pos)
 		return
 	}
 	match sym.kind {

vlib/v/checker/tests/import_symbol_private_err.out

@@ -52,7 +52,7 @@ vlib/v/checker/tests/import_symbol_private_err.vv:12:4: error: method `map[strin
       |       ~~~~~~~~~~~
    13 |     _ = ReaderWriterImpl{}
    14 | }
-vlib/v/checker/tests/import_symbol_private_err.vv:13:6: error: type `io.ReaderWriterImpl` is private
+vlib/v/checker/tests/import_symbol_private_err.vv:13:6: error: struct `io.ReaderWriterImpl` was declared as private to module `io`, so it can not be used inside module `main`
    11 |         'h': 2
    12 |     }.exists('h')
    13 |     _ = ReaderWriterImpl{}

vlib/v/checker/tests/modules/module_with_redeclaration.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/modules/module_with_redeclaration/redeclare_time_structs.v:1:1: error: project must include a `main` module or be a shared library (compile with `v -shared`)
+    1 | module module_with_redeclaration
+      | ^
+    2 | 
+    3 | pub const used = 1

vlib/v/checker/tests/modules/module_with_redeclaration/redeclare_time_structs.v

@@ -0,0 +1,8 @@
+module module_with_redeclaration
+
+pub const used = 1
+
+struct C.timeval {
+	tv_sec  u64 // Seconds.
+	tv_usec u64 // Microseconds.
+}

vlib/v/checker/tests/private_redeclaration_of_C_timeval.out

@@ -0,0 +1,14 @@
+vlib/time/time.c.v:90:11: error: struct `C.timeval` was declared as private to module `module_with_redeclaration`, so it can not be used inside module `time`
+   88 |         return C.GetTickCount()
+   89 |     } $else {
+   90 |         ts := C.timeval{}
+      |                 ~~~~~~~~~
+   91 |         C.gettimeofday(&ts, 0)
+   92 |         return i64(ts.tv_sec * u64(1000) + (ts.tv_usec / u64(1000)))
+vlib/v/checker/tests/private_redeclaration_of_C_timeval.vv:9:10: error: struct `C.timeval` was declared as private to module `module_with_redeclaration`, so it can not be used inside module `main`
+    7 |     dump(time.now()) // ensure `time` is used
+    8 |     //
+    9 |     ts := C.timeval{}
+      |             ~~~~~~~~~
+   10 |     dump(ts)
+   11 | }

vlib/v/checker/tests/private_redeclaration_of_C_timeval.vv

@@ -0,0 +1,11 @@
+import module_with_redeclaration as mr
+import time
+
+const used = mr.used
+
+fn main() {
+	dump(time.now()) // ensure `time` is used
+	//
+	ts := C.timeval{}
+	dump(ts)
+}

vlib/v/checker/tests/struct_type_is_private_err.out

@@ -3,7 +3,7 @@ vlib/v/checker/tests/struct_type_is_private_err.vv:1:8: warning: module 'sqlite'
       |        ~~~~~~
     2 | 
     3 | fn main(){
-vlib/v/checker/tests/struct_type_is_private_err.vv:4:10: error: type `C.sqlite3` is private
+vlib/v/checker/tests/struct_type_is_private_err.vv:4:10: error: struct `C.sqlite3` was declared as private to module `sqlite`, so it can not be used inside module `main`
     2 | 
     3 | fn main(){
     4 |     _ := &C.sqlite3{}