replace leading _ for private symbols with keyword hide

examples/packages/hidden_invalid/app/a.bro

@@ -1,13 +1,13 @@
-_sibling func() i32 {
+hide sibling func() i32 {
 	return 1
 }
 
-_Sibling :: struct {
+hide Sibling :: struct {
 	value i32
 }
 
-_sibling_value :: 1
+hide sibling_value :: 1
 
-_collision c_func() i32 {
+collision c_func() i32 {
 	return 1
 }

examples/packages/hidden_invalid/app/b.bro

@@ -1,17 +1,17 @@
 import "../dep"
 
-_collision func() i32 {
+hide collision func() i32 {
 	return 2
 }
 
-read_sibling func(value _Sibling) i32 {
-	return value.value + _sibling_value
+read_sibling func(value Sibling) i32 {
+	return value.value + sibling_value
 }
 
 read_sibling_value func() i32 {
-	return _sibling_value
+	return sibling_value
 }
 
 main func() i32 {
-	return _sibling() + dep._secret() + read_sibling(_Sibling { value = 1 }) + read_sibling_value()
+	return sibling() + dep.secret() + read_sibling(Sibling { value = 1 }) + read_sibling_value()
 }