Fix fetch public/fastboot-fetch.js module definition for Fastboot

[xg-wang]

Fix #166, depends on #171

In v6.2.0 fetch/setup was added to public/fastboot-fetch.js so we can setup fetch module with fastboot request info. However this file will be overwritten by any addon's dependency ember-fetch based on last-write-win. To fix this:

• Rename public/fastboot-fetch.js to public/fetch-fastboot.js to avoid
  lower version ember-fetch overwrite this file.
• Overrides treeForPublic in index.js to only include public tree if top
  level addon.

For example, top level addon ember-fetch is 6.2.1, transitive dependency is anything before 6.2.0 will result in this dist:

dist
├── assets
│   ├── ...
├── ember-fetch
│   ├── fastboot-fetch.js
│   └── fetch-fastboot.js

6.2.1 as top level addon will manifest.vendorFiles.push('ember-fetch/fetch-fastboot.js'); so the fastboot.js will just be ignored.

This commit also defines fetch module with a setupFastboot method to set host and
protocol for every instance. This could avoid module redefinition.

[Turbo87]

What if ember-fetch is never the top level addon? What if it is only used as a subdep with several versions?

[xg-wang]

@Turbo87 updateFastBootManifest is only called on top level addons. For subdeps only treeForVendor has effect, and we don't calculate cache key for vendor.

So if ember-fetch is only a subdep it only works for browser, and won't have ember-fetch/xx.js in dist/

[Turbo87]

@xg-wang this PR seems to contain a lot of unrelated changes now. can you make sure to open dedicated PRs for those unrelated changes?

[xg-wang]

@Turbo87 This PR is now only relying on #171.

[samselikoff]

#171 is merged, this good to go?

[runspired]

We may want to rethink this design a bit now that ember-fetch is included as a dependency by ember-data

[rwjblue]

Agree

[dnalagatla]

+1