Building from the work started by @nlf

ref: https://gist.github.com/nlf/52ca6adab49e5b3939ba37c7f0fc51c6

Co-authored-by: nlf <quitlahok@gmail.com>

- Added param checks and throws TypeErrors on unexpected usage
- Simplified a bit some of the logic aroung builtin modules cache
- Fixed some errors introduced while porting the original work from @nlf

The mock keys should now be paths relative to the current script/tests
that is defining it.

Add support to modules that instant executing a required module.

    require('./something')()

This mock-fn-as-callback-style was used in test found in npm/cli:

    t.test('mock as callback style', t => {
      t.mock('../my-module.js', {
        '../sub-module.js': arg => {
          t.equal(arg, 'expected')
          t.end()
        }
      })
    })

Also added more varied situations to stress test the module replacement,
such as using t.mock within one of the defined mocks, require calls at
execution time along with tests for the mock as callback style.