Derive macro doesn't support type aliases for the database

We should support the code below:
```rs
struct MyStorage { ... foo: SingletonStorage ... }
struct Foo;
type MyDb = inc_complete::DbHandle;

#[intermediate(id = 1)]
fn foo(ctx: &incremental::Parse, db: &MyDb) -> i32 {
0
}
```
But we get an error when doing so:
```
error: DbHandle must have a generic type parameter for the storage type
--> src/foo/mod.rs:84:45
|
84 | fn foo(ctx: &incremental::Parse, db: &incremental::DbHandle) -> Arc {
| ^^^^^^^^^^^^^^^^^^^^^
```
If we name the alias something else we get an error making it look like the macro is looking for an exact name:
```
error: second parameter must be &DbHandle
--> src/foo/mod.rs:86:45
|
86 | fn foo(ctx: &incremental::Parse, db: &Foo) -> Arc {
| ^^^
```
This is overly restrictive and prevents users from using the derive macros in more situations

Start by locating the derive macro's handling of the second parameter and how it validates DbHandle's storage type. Reproduce the issue with the MyDb alias and with the differently named alias, then add coverage showing both forms are accepted while preserving the intended storage type check.

Written by the indexing model from the issue text.