Fixed + more consistent basedir handling

[goto-bus-stop]

This is a 🐛 bug fix

When the entry point is given as a directory, the build destination
dist and asset paths would be calculated relative to the parent
directory. With this patch, the default build destination is ./dist
relative to the entry point file, and assets are served from ./assets
relative to the entry point file.

Ensures that these:

bankai build example/index.js
bankai build example

both do the same thing.

Checklist

• tests pass
• tests and/or benchmarks are included

Context

14:14 < louisc> I'm having an issue with bankai whereby if I try and
  access a file in the `assets` directory, bankai will try and serve
  the asset from the wrong filepath
14:15 < louisc> for example, if I'm running bankai from ~/dev/mywebsite,
  it will try and load assets from ~/dev/assets, rather than
  ~/dev/mywebsite/assets
14:15 < louisc> I'm using v9.10.7
14:16 < louisc> I'm wondering if this is to do with the change in
  9.10.4 that dealt with asset pathing?
14:17 < louisc> cc benlyn goto-bus-stop
14:18 < goto-bus-stop> louisc: o, possibly
14:19 < goto-bus-stop> louisc: does it work correctly if you do
  `bankai start ~/dev/mywebsite/index.js` instead?
14:19 < louisc> goto-bus-stop: yes

Semver

Patch