Support expansion of setup modes for find_hooks()

[uwiger]

From the docs:

Using the setup environment variable modes, it is possible to
define a mode that includes all hooks from different modes.
The format is [{M1, [M2,...]}]. The expansion is done recursively,
so a mode entry in the right-hand side of a pair can expand into other
modes. In order to be included in the final list of modes, an expanding
mode needs to include itself in the right-hand side. For example:

• Applying a to [{a, [b]}] returns [b]
• Applying a to [{a, [a,b]}] returns [a,b]
• Applying a to [{a, [a,b]},{b,[c,d]}] returns [a,c,d]

A typical application of this would be [{test, [normal, test]}], where
starting in the test mode would cause all normal and all test hooks
to be executed.

[tolbrino]

@uwiger :+1 for the test coverage

[tolbrino]

@uwiger Can be merged from my perspective.

[uwiger]

Thanks. Merged.