parser: add an error for `import mod.sub as mod` and `import mod as mod` (#6194)

examples/net_raw_http.v

@@ -1,5 +1,5 @@
 // Simple raw HTTP head request
-import x.net as net
+import x.net
 import time
 
 // Make a new connection

vlib/math/stats/stats_test.v

@@ -1,4 +1,4 @@
-import math.stats as stats
+import math.stats
 import math
 
 fn test_freq() {

vlib/v/checker/tests/import_mod_as_mod_err.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/import_mod_as_mod_err.v:1:16: error: import alias `math as math` is redundant
+    1 | import math as math
+      |                ~~~~
+    2 |
+    3 | fn main() {

vlib/v/checker/tests/import_mod_as_mod_err.vv

@@ -0,0 +1,5 @@
+import math as math
+
+fn main() {
+    println(math.e)
+}

vlib/v/checker/tests/import_mod_sub_as_sub_err.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/import_mod_sub_as_sub_err.v:1:25: error: import alias `encoding.utf8 as utf8` is redundant
+    1 | import encoding.utf8 as utf8
+      |                         ~~~~
+    2 |
+    3 | fn main() {

vlib/v/checker/tests/import_mod_sub_as_sub_err.vv

@@ -0,0 +1,5 @@
+import encoding.utf8 as utf8
+
+fn main() {
+    println(utf8.validate_str('añçá'))
+}

vlib/v/parser/parser.v

@@ -1495,6 +1495,9 @@ fn (mut p Parser) import_stmt() ast.Import {
 	if p.tok.kind == .key_as {
 		p.next()
 		mod_alias = p.check_name()
+		if mod_alias == mod_name.split('.').last() {
+			p.error_with_pos('import alias `$mod_name as $mod_alias` is redundant', p.prev_tok.position())
+		}
 	}
 	node := ast.Import{
 		pos: pos

vlib/x/net/udp_test.v

@@ -1,4 +1,4 @@
-import x.net as net
+import x.net
 import time
 
 fn echo_server(_c net.UdpConn) {