Add FAQ for debugging source configuration

[Robert P. Goldman]

Add to the ASDF manual an FAQ describing what to do if ASDF can't find a system that you think it should find.

Faré writes:

> You can
> (alexandria:hash-table-alist asdf/source-registry::*source-registry*)
>
> or for a higher-level view, you can
> (asdf/source-registry:flatten-source-registry)

Add the flattening function to the function index of the manual and put in a FAQ.

[Robert P. Goldman (rpgoldman)]

Another suggestion from Faré:

(sort (loop for p being the hash-values of asdf::*source-registry*
collect p) 'string< :key 'pathname-name)

[Faré (fahree)]

Note that flatten-source-registry is NOT exported from asdf: and for good reason.

It *is* exported from the current version of asdf/source-registry: though, and for good reason (so it's accessible as asdf:: )