Bicep restore should restore modules concurrently

Module restore is currently done sequentially. We need to do it concurrently with some limit on how many can be done at the same time. This will improve restore latency.

Start by locating the module restore entry point and tracing how modules are currently restored sequentially. Define a bounded concurrency approach and verify that the restore path limits simultaneous work while reducing restore latency; the issue does not name specific files or tests.

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