Implement SubcommandsProvider

[imunproductive]

This PR adds a SubcommandsProvider, a way to easily collect all the commands from PATH with specific prefix.

@race-of-sloths include

[artob]

Alphabetic ordering in these, always

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Current status: executed

Reviewer	Score
@artob	8

Your contribution is much appreciated with a final score of 8!
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[artob]

@race-of-sloths score 8

[artob]

LGTM with some minor nits I will address after the merge.