Type aliases not getting referenced using --strictNullChecks with union types

[HoldYourWaffle]

Possibly related to #232.

Given the following TypeScript:

type NotNull = string;
type Null = null;

interface Type {
	neverNull: NotNull;
	alwaysNull: Null;
	nullable: NotNull | Null;
}

The following schema is generated using only --strictNullChecks:

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "properties": {
        "alwaysNull": { "type": "null" },
        "notnullable": { "type": "string" },
        "nullable": {
            "type": [ "null", "string" ]
        }
    },
    "type": "object"
}

So far so good. But when I add --aliasRefs I get the following:

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "definitions": {
        "NotNull": {
            "type": "string"
        },
        "Null": {
            "type": "null"
        }
    },
    "properties": {
        "alwaysNull": { "$ref": "#/definitions/Null" },
        "notnullable": { "$ref": "#/definitions/NotNull" },
        "nullable": {
            "type": [
                "null",
                "string"
            ]
        }
    },
    "type": "object"
}

As you can see the nullable property doesn't reference the NotNull and Null types. This example is of course not a real-world scenario, but it's the easiest way to demonstrate the bug.

Interestingly, when I create a type Nullable = Null | NotNull and use that for a property the interface will reference the Nullable type, but the Nullable definition will not reference Null or NotNull:

type NotNull = string;
type Null = null;
type Nullable = Null | NotNull;

interface Type {
	nullableAliased: Nullable;
}

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "definitions": {
        "Nullable": {
            "type": [
                "null",
                "string"
            ]
        }
    },
    "properties": {
        "nullableAliased": {
            "$ref": "#/definitions/Nullable"
        }
    },
    "type": "object"
}