Changes required for Embroider build to support Fastboot apps

[SparshithNR]

This PR address the review comments from #729

[dnalagatla]

Thank you @SparshithNR for working on updating code based on new review comments. The change looks good to me.

[kratiahuja]

LGTM overall, except the concern on wanting to understand the data-fastboot-ignore usecase more.

[kratiahuja]

Will merge once CI is completed.

[ef4]

This is still an important issue, but after much more end-to-end testing of full apps with fastboot & embroider I don't actually think this is the right solution.

Instead of mangling the build output in postBuild -- a step I can't really guarantee will always even exists -- the core code in this PR belongs in https://github.com/ember-fastboot/fastboot instead, where it can discover the right assets when the Fastboot instance is reading from distPath.

Also, the earlier reverted version of this feature is still present in the latest release because no release has been cut since it was reverted.

[kiwiupover]

@dnalagatla @ef4 and @rwjblue what needs to happen to this PR before we can cut an embroider version of ember-cli-fastboot

[ef4]

This PR is superseded, the equivalent changes were already made in fastboot itself.

We probably do need a release of ember-cli-fastboot that upgrades to that newer fastboot.

There is also work to do here to make ember-cli-fastboot natively emit the new format accepted by fastboot, although that is not a blocker because fastboot accepts all older formats too.