Package Mode: Use aliases when used in source

[JacobOaks]

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

e.g.,
some package:

package somgpkg

import "somepkg/internal/apicodec"
...
type Codec = apicodec.Codec

mockgen input:

type Foo interface{
	Bar() somepkg.Codec
}

mock:

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.

The nice exception to the side effect is when we explicitly request mock generation for an alias type, since at that point we are dealing with the object, not the type.

With this PR, the mocks get generated correctly now:

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.