[6.0.0] '--rulesdir' option doesn't work if a config file has 'root:true'

[mysticatea]

Tell us about your environment

• ESLint Version: 6.0.0
• Node Version: 12.2.0
• npm Version: 6.9.0

What parser (default, Babel-ESLint, etc.) are you using?

Default.

Please show your full configuration:

module.exports = {
    root: true,
    rules: {
        test: "error"
    }
};

// internal-rules/test.js
const { Linter } = require("eslint")
const linter = new Linter()
const rules = linter.getRules()

module.exports = rules.get("no-console")

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

console.log("hello")

eslint test.js --rulesdir internal-rules

What did you expect to happen?

The test rule in the internal-rules directory warns the code.

What actually happened? Please include the actual, raw output from ESLint.

C:\Users\t-nagashima.AD\dev\sandbox\test.js
  1:1  error  Definition for rule 'test' was not found  test

✖ 1 problem (1 error, 0 warnings)

Are you willing to submit a pull request to fix this bug?

Yes.