mw.loader: Remove Deferred overhead from execute() hot code path

mw.loader: Remove Deferred overhead from execute() hot code path

This was introduced by 0ac4f99804c for the concept of globally
implied legacy dependencies.

Commit ba257035b07 then re-purposed this for the site/user module
dependency.

The legacy wait was removed in 0ac6076b4c0, thus leaving only the
site/user code.

This commit now simplifies the code back to how it was before 0ac4f99804c
(but keeping the improved error handling from ba257035b07).

Change-Id: I2eae3b78fbe4f03a7a098d8a6233bdc2b79171b8