checker: include import aliases when checking for import duplicates (#18450)

vlib/v/checker/checker.v

@@ -179,6 +179,12 @@ pub fn (mut c Checker) check(ast_file_ &ast.File) {
 			if ast_import.mod == ast_file.imports[j].mod {
 				c.error('`${ast_import.mod}` was already imported on line ${
 					ast_file.imports[j].mod_pos.line_nr + 1}', ast_import.mod_pos)
+			} else if ast_import.mod == ast_file.imports[j].alias {
+				c.error('`${ast_file.imports[j].mod}` was already imported as `${ast_import.alias}` on line ${
+					ast_file.imports[j].mod_pos.line_nr + 1}', ast_import.mod_pos)
+			} else if ast_import.alias == ast_file.imports[j].alias {
+				c.error('`${ast_file.imports[j].mod}` was already imported on line ${
+					ast_file.imports[j].alias_pos.line_nr + 1}', ast_import.alias_pos)
 			}
 		}
 	}

vlib/v/checker/tests/import_mod_as_import_alias_duplicate_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/import_mod_as_import_alias_duplicate_err.vv:2:19: error: `json` was already imported on line 1
+    1 | import json
+    2 | import x.json2 as json
+      |                   ~~~~
+    3 | 
+    4 | numbers := {

vlib/v/checker/tests/import_mod_as_import_alias_duplicate_err.vv

@@ -0,0 +1,12 @@
+import json
+import x.json2 as json
+
+numbers := {
+	'one':   1
+	'two':   2
+	'three': 3
+	'four':  4
+}
+
+out := json.encode(numbers)
+println(out)

vlib/v/checker/tests/import_mod_as_import_duplicate_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/import_mod_as_import_duplicate_err.vv:2:8: error: `x.json2` was already imported as `json` on line 1
+    1 | import x.json2 as json
+    2 | import json
+      |        ~~~~
+    3 | 
+    4 | numbers := {

vlib/v/checker/tests/import_mod_as_import_duplicate_err.vv

@@ -0,0 +1,12 @@
+import x.json2 as json
+import json
+
+numbers := {
+	'one':   1
+	'two':   2
+	'three': 3
+	'four':  4
+}
+
+out := json.encode(numbers)
+println(out)