Don't require dependencies to be in the scripts section

[aomarks]

We're quite strict about every dependency being a real npm script, but sometimes it's nice to have intermediate scripts that are purely defined in the wireit section, even if they can't be invoked directly. This should be allowed.

[filimon-danopoulos-stratsys]

This might be a bit backwards in relation to this issue but I believe it is related.

Assuming I can have "pure" wireit scripts I would like the ability to run the same scripts too but without having to expose them in scripts. That way my package.json scripts section can be kept very high level, build, lint, test etc without having to care about specifics and dependencies between scripts.

{ 
  "scripts": {
    "build": "yarn wireit:build",
  },
  "wireit": {
    "wireit:build": {
      "command": "esbuild src/index.ts",
      "clean": "if-file-deleted",
      "files": [
        "./src/**/*"
      ],
      "output": [
        "./dist/**"
      ],
      "dependencies": [
        {
          "script": "wireit:tsc",
          "cascade": false
        }
      ]
    },
    "wireit:tsc": {
      "command": "tsc --project tsconfig.json",
      "clean": true,
      "files": [
        "./src/**/*",
        "tsconfig.json"
      ],
      "output": [
        "./typings/**/*"
      ]
    }
  },
}

However when need I would like to tap in to the wireit scripts too, wireit run wireit:tsc.