Add checks for conflicts for aliases

- Check that alias is not the same as name of other field (it still can be the name of owning field/variant)
- Check that aliases are unique, i. e. two different fields does not use the same alias

test_suite/tests/ui/conflict/alias.rs

@@ -0,0 +1,40 @@
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+struct S1 {
+    /// Expected error on "alias b", because this is a name of other field
+    /// Error on "alias a" is not expected because this is a name of this field
+    /// Error on "alias c" is not expected because field `c` is skipped
+    #[serde(alias = "a", alias = "b", alias = "c")]
+    a: (),
+
+    /// Expected error on "alias c", because it is already used as alias of `a`
+    #[serde(alias = "c")]
+    b: (),
+
+    #[serde(skip_deserializing)]
+    c: (),
+}
+
+#[derive(Deserialize)]
+struct S2 {
+    /// Expected error on "alias c", because this is a name of other field after
+    /// applying rename rules
+    #[serde(alias = "b", alias = "c")]
+    a: (),
+
+    #[serde(rename = "c")]
+    b: (),
+}
+
+#[derive(Deserialize)]
+#[serde(rename_all = "UPPERCASE")]
+struct S3 {
+    /// Expected error on "alias B", because this is a name of field after
+    /// applying rename rules
+    #[serde(alias = "B", alias = "c")]
+    a: (),
+    b: (),
+}
+
+fn main() {}