Optimize `bst build` initialization time

Background

bst shell and bst build can currently take a few seconds to initialize (because we parse bst files, etc.), which is very interrupting in an edit-compile cycle. This is apparently bad enough that our users are hacking around buildstream and following unsupported workflows just to speed up initialization.

Task description

We need to optimize this.

It's probably also worth looking at #703 (closed) at the same time as this.

It's probably also worth looking at #712 (closed) at the same time as this.

Acceptance Criteria

• BuildStream initialization time (time from typing bst build x to x showing build output with no fetching/tracking) should be minimal, preferably sub-second times.
• Timing Comparison before and after applying the solution should be provided

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