checker: do not allow module aliases started with '_' (underscore) (#9588)

vlib/v/checker/checker.v

@@ -3977,6 +3977,7 @@ fn (mut c Checker) hash_stmt(mut node ast.HashStmt) {
 }
 
 fn (mut c Checker) import_stmt(imp ast.Import) {
+	c.check_valid_snake_case(imp.alias, 'module alias', imp.pos)
 	for sym in imp.syms {
 		name := '${imp.mod}.$sym.name'
 		if sym.name[0].is_capital() {

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

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/modules/module_alias_started_with_underscore/main.v:3:1: error: module alias `_` cannot start with `_`
+    1 | module main
+    2 | 
+    3 | import underscore as _
+      | ~~~~~~~~~~~~~~~~~~~~~~
+    4 | 
+    5 | fn main() {

vlib/v/checker/tests/modules/module_alias_started_with_underscore/main.v

@@ -0,0 +1,7 @@
+module main
+
+import underscore as _
+
+fn main() {
+	_.foo()
+}

vlib/v/checker/tests/modules/module_alias_started_with_underscore/underscore.v

@@ -0,0 +1,5 @@
+module underscore
+
+pub fn foo() {
+	println('bar')
+}