fix: specify './package.json' in 'exports'

[tchupp]

Some javascript bundlers use require.resolve to identify the package.json. If the package.json can't be resolved, the dependency isn't included.

Specifically in my case, generate-package-json-webpack-plugin was having trouble locating the package.json due to the export fields.

Test case, before:

$ npm i graphql

$ npm i graphql-middleware

$ node

> require.resolve("graphql/package.json", { paths: ["/Users/theochupp/Documents/graphql-middleware-test/apps/graphql/src"], })
'/Users/theochupp/Documents/graphql-middleware-test/node_modules/graphql/package.json'

> require.resolve("graphql-middleware/package.json", { paths: ["/Users/theochupp/Documents/graphql-middleware-test/apps/graphql/src"], })
Uncaught:
Error: Cannot find module '/Users/theochupp/Documents/graphql-middleware-test/node_modules/graphql-middleware/dist/package.json.js'
    at createEsmNotFoundErr (internal/modules/cjs/loader.js:842:15)
    at finalizeEsmResolution (internal/modules/cjs/loader.js:835:15)
    at resolveExports (internal/modules/cjs/loader.js:424:14)
    at Function.Module._findPath (internal/modules/cjs/loader.js:464:31)
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:802:27)
    at Function.resolve (internal/modules/cjs/helpers.js:80:19) {
  code: 'MODULE_NOT_FOUND',
  path: '/Users/theochupp/Documents/graphql-middleware-test/node_modules/graphql-middleware/package.json'
}

Test case, after:

$ npm i graphql

$ npm i graphql-middleware

$ node

> require.resolve("graphql/package.json", { paths: ["/Users/theochupp/Documents/graphql-middleware-test/apps/graphql/src"], })
'/Users/theochupp/Documents/graphql-middleware-test/node_modules/graphql/package.json'

> require.resolve("graphql-middleware/package.json", { paths: ["/Users/theochupp/Documents/graphql-middleware-test/apps/graphql/src"], })
'/Users/theochupp/Documents/graphql-middleware-test/node_modules/graphql-middleware/package.json'

I'm not sure this is even worth adding a unit test for 😂