v0.5.0 included uber-go#207, which replaced reflect mode with package mode.
One issue with package mode that came up (ref: uber-go#216) was that
generated mocks for interfaces that referred to alias types
were referring to the aliases' underlying names instead.

e.g.,
source:
```go
import "github.com/tikv/client-go/v2/internal/apicodec"
...
type Codec = apicodec.Codec

type Foo interface{
	Bar() Codec
}
```
mock:
```go
func (m *MockFoo) Bar() apicodec.Codec { // This is a problem, since apicodec is an internal package.
    // ...
}
```

While technically this problem is solved in Go 1.23 with explicit alias types representation,
(indeed, if you run mockgen on the example in the linked issue with
`GODEBUG=gotypesalias=1`, you get the expected behavior)
since we support the last two versions, we can't bump `go.mod` to 1.23 yet.
This leaves us with the old behavior, where `go/types` does not track alias types.
You can tell if an object is an alias, but not a type itself,
and there is no way to retrieve the object of interest
at the point where we are recursively parsing method types.

This PR works around this issue (temporarily) by using syntax information
to find all references to aliases in the source package.
When we find one, we record it in a mapping of underlying type -> alias name.
Later, while we parse the type tree, we replace any underlying types
in the mapping with their alias names.

The unexpected side effect of this is that _all_ references to the underlying type
in the generated mocks will be replaced with the alias, even if
the source used the underlying name. This is fine because:
* If the alias is in the mapping, it was used at least once, which means its accessible.
* From a type-checking perspective, aliases and their underlying types are equivalent.

With this PR, the mocks get generated correctly now:
```go
func (m *MockFoo) Bar() Codec {
    // ...
}
```

Once we can bump `go.mod` to 1.23, we should definitely remove this,
since the new type alias type nodes solve this problem automatically.