Default #src is misconfigured

The blueprint is confused about the purpose of #src. If you only ever use it in tests, it works, because there it's always supposed to actually mean the (unbuilt) src directory.

But if you mention `#src` in published code, the build incorrectly leaves it alone and points the actual consuming app at src instead of dist.

This is probably the root cause of https://github.com/ember-cli/ember-addon-blueprint/issues/138 too.

`#src` is not a solution to the problem of built and unbuilt being different. I continue to think we should use a custom export condition instead.