all: do not warn/error for import flag as _

vlib/v/checker/checker.v

@@ -458,7 +458,7 @@ fn (mut c Checker) check_valid_snake_case(name string, identifier string, pos to
 	if c.pref.translated || c.file.is_translated {
 		return
 	}
-	if !c.pref.is_vweb && name.len > 0 && (name[0] == `_` || name.contains('._')) {
+	if !c.pref.is_vweb && name.len > 1 && (name[0] == `_` || name.contains('._')) {
 		c.error('${identifier} `${name}` cannot start with `_`', pos)
 	}
 	if !c.pref.experimental && util.contains_capital(name) {

vlib/v/checker/tests/import_unused_warning.out

@@ -1,5 +1,6 @@
-vlib/v/checker/tests/import_unused_warning.vv:1:8: warning: module 'time' is imported but never used 
-    1 | import time
+vlib/v/checker/tests/import_unused_warning.vv:2:8: warning: module 'time' is imported but never used
+    1 | import os as _
+    2 | import time
       |        ~~~~
-    2 | fn main() {
-    3 |     println('hello, world')
+    3 | 
+    4 | fn main() {

vlib/v/checker/tests/import_unused_warning.vv

@@ -1,4 +1,6 @@
+import os as _
 import time
+
 fn main() {
 	println('hello, world')
 }

vlib/v/checker/tests/incorrect_name_module.out

@@ -1,3 +1,10 @@
+vlib/v/checker/tests/incorrect_name_module.vv:4:8: warning: module 'os' is imported but never used
+    2 | 
+    3 | import math as _
+    4 | import os
+      |        ~~
 vlib/v/checker/tests/incorrect_name_module.vv:1:1: error: module name `_A` cannot start with `_`
     1 | module _A
-      | ~~~~~~~~~
+      | ~~~~~~~~~
+    2 | 
+    3 | import math as _

vlib/v/checker/tests/incorrect_name_module.vv

@@ -1 +1,4 @@
 module _A
+
+import math as _
+import os

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

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

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

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

vlib/v/gen/c/testdata/unused_import_aliased_as_underscore.c.must_have

@@ -0,0 +1,3 @@
+struct flag__Flag {
+println(flag__FlagParser_usage(fs));
+void flag__FlagParser_footer(flag__FlagParser* fs, string footer) {

vlib/v/gen/c/testdata/unused_import_aliased_as_underscore.vv

@@ -0,0 +1 @@
+import flag as _

vlib/v/parser/module.v

@@ -58,7 +58,7 @@ fn (mut p Parser) check_unused_imports() {
 	for import_m in p.ast_imports {
 		alias := import_m.alias
 		mod := import_m.mod
-		if !p.is_used_import(alias) {
+		if !(alias.len == 1 && alias[0] == `_`) && !p.is_used_import(alias) {
 			mod_alias := if alias == mod { alias } else { '${alias} (${mod})' }
 			p.warn_with_pos("module '${mod_alias}' is imported but never used", import_m.mod_pos)
 		}