Minimal example

.gitignore

out.mjs
node_modules/

index.js

import * as joi from './joi';

const out = { joi };

export {
  out as default
}

joi.js

const Schemas = require('./schemas');

exports = module.exports = {};

exports.object = function() {
  return {};
};

exports.check = function() {
  return Schemas.check();
};

package-lock.json

{
  "name": "rollup-joi",
  "version": "1.0.0",
  "lockfileVersion": 1,
  "requires": true,
  "dependencies": {
    "@rollup/plugin-commonjs": {
      "version": "11.0.0",
      "resolved": "https://registry.npmjs.org/@rollup/plugin-commonjs/-/plugin-commonjs-11.0.0.tgz",
      "integrity": "sha512-jnm//T5ZWOZ6zmJ61fReSCBOif+Ax8dHVoVggA+d2NA7T4qCWgQ3KYr+zN2faGEYLpe1wa03IzvhR+sqVLxUWg==",
      "dev": true,
      "requires": {
        "@rollup/pluginutils": "^3.0.0",
        "estree-walker": "^0.6.1",
        "is-reference": "^1.1.2",
        "magic-string": "^0.25.2",
        "resolve": "^1.11.0"
      }
    },
    "@rollup/pluginutils": {
      "version": "3.0.1",
      "resolved": "https://registry.npmjs.org/@rollup/pluginutils/-/pluginutils-3.0.1.tgz",
      "integrity": "sha512-PmNurkecagFimv7ZdKCVOfQuqKDPkrcpLFxRBcQ00LYr4HAjJwhCFxBiY2Xoletll2htTIiXBg6g0Yg21h2M3w==",
      "dev": true,
      "requires": {
        "estree-walker": "^0.6.1"
      }
    },
    "@types/estree": {
      "version": "0.0.39",
      "resolved": "https://registry.npmjs.org/@types/estree/-/estree-0.0.39.tgz",
      "integrity": "sha512-EYNwp3bU+98cpU4lAWYYL7Zz+2gryWH1qbdDTidVd6hkiR6weksdbMadyXKXNPEkQFhXM+hVO9ZygomHXp+AIw==",
      "dev": true
    },
    "@types/node": {
      "version": "13.1.2",
      "resolved": "https://registry.npmjs.org/@types/node/-/node-13.1.2.tgz",
      "integrity": "sha512-B8emQA1qeKerqd1dmIsQYnXi+mmAzTB7flExjmy5X1aVAKFNNNDubkavwR13kR6JnpeLp3aLoJhwn9trWPAyFQ==",
      "dev": true
    },
    "acorn": {
      "version": "7.1.0",
      "resolved": "https://registry.npmjs.org/acorn/-/acorn-7.1.0.tgz",
      "integrity": "sha512-kL5CuoXA/dgxlBbVrflsflzQ3PAas7RYZB52NOm/6839iVYJgKMJ3cQJD+t2i5+qFa8h3MDpEOJiS64E8JLnSQ==",
      "dev": true
    },
    "estree-walker": {
      "version": "0.6.1",
      "resolved": "https://registry.npmjs.org/estree-walker/-/estree-walker-0.6.1.tgz",
      "integrity": "sha512-SqmZANLWS0mnatqbSfRP5g8OXZC12Fgg1IwNtLsyHDzJizORW4khDfjPqJZsemPWBB2uqykUah5YpQ6epsqC/w==",
      "dev": true
    },
    "is-reference": {
      "version": "1.1.4",
      "resolved": "https://registry.npmjs.org/is-reference/-/is-reference-1.1.4.tgz",
      "integrity": "sha512-uJA/CDPO3Tao3GTrxYn6AwkM4nUPJiGGYu5+cB8qbC7WGFlrKZbiRo7SFKxUAEpFUfiHofWCXBUNhvYJMh+6zw==",
      "dev": true,
      "requires": {
        "@types/estree": "0.0.39"
      }
    },
    "magic-string": {
      "version": "0.25.4",
      "resolved": "https://registry.npmjs.org/magic-string/-/magic-string-0.25.4.tgz",
      "integrity": "sha512-oycWO9nEVAP2RVPbIoDoA4Y7LFIJ3xRYov93gAyJhZkET1tNuB0u7uWkZS2LpBWTJUWnmau/To8ECWRC+jKNfw==",
      "dev": true,
      "requires": {
        "sourcemap-codec": "^1.4.4"
      }
    },
    "path-parse": {
      "version": "1.0.6",
      "resolved": "https://registry.npmjs.org/path-parse/-/path-parse-1.0.6.tgz",
      "integrity": "sha512-GSmOT2EbHrINBf9SR7CDELwlJ8AENk3Qn7OikK4nFYAu3Ote2+JYNVvkpAEQm3/TLNEJFD/xZJjzyxg3KBWOzw==",
      "dev": true
    },
    "resolve": {
      "version": "1.14.1",
      "resolved": "https://registry.npmjs.org/resolve/-/resolve-1.14.1.tgz",
      "integrity": "sha512-fn5Wobh4cxbLzuHaE+nphztHy43/b++4M6SsGFC2gB8uYwf0C8LcarfCz1un7UTW8OFQg9iNjZ4xpcFVGebDPg==",
      "dev": true,
      "requires": {
        "path-parse": "^1.0.6"
      }
    },
    "rollup": {
      "version": "1.27.14",
      "resolved": "https://registry.npmjs.org/rollup/-/rollup-1.27.14.tgz",
      "integrity": "sha512-DuDjEyn8Y79ALYXMt+nH/EI58L5pEw5HU9K38xXdRnxQhvzUTI/nxAawhkAHUQeudANQ//8iyrhVRHJBuR6DSQ==",
      "dev": true,
      "requires": {
        "@types/estree": "*",
        "@types/node": "*",
        "acorn": "^7.1.0"
      }
    },
    "sourcemap-codec": {
      "version": "1.4.6",
      "resolved": "https://registry.npmjs.org/sourcemap-codec/-/sourcemap-codec-1.4.6.tgz",
      "integrity": "sha512-1ZooVLYFxC448piVLBbtOxFcXwnymH9oUF8nRd3CuYDVvkRBxRl6pB4Mtas5a4drtL+E8LDgFkQNcgIw6tc8Hg==",
      "dev": true
    }
  }
}

package.json

{
  "name": "rollup-joi",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "build": "rollup -c rollup.config.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {},
  "devDependencies": {
    "@rollup/plugin-commonjs": "^11.0.0",
    "rollup": "^1.27.14"
  }
}

rollup.config.js

import commonjs from '@rollup/plugin-commonjs';

export default {
  input: 'index.js',
  output: {
    file: 'out.mjs',
    format: 'es'
  },
  plugins: [
    commonjs()
  ]
};

schemas.js

const Joi = require('./joi');

const options = {
  foo: Joi.object()
}

exports.schemas = {
  default: options
};

smoke.html

<!doctype html>
<html lang="en">
<script type="module">
  import * as main from './out.mjs';

  console.log(main);
</script>