tsconfig gets packed to individual packages

Visual Studio code shows compilation errors for `node_modules` folder if a package from `@googleapis/` is installed:

The reason for that is that those packages include `tsconfig.json` which extends a (not installed) `gts` config, which wouldn't exist:

We should include `files:` section to the generated `package.json` for `@googleapis/` packages and include only files that are needed there.