refactor: factorize external executables

[pharcosyle]

I'd like to see external executable dependencies (ripgrep, etc.) better consolidated in the code.

Currently they're scattered all over the place. Ideally there would be a single defvar/defcustom for each that would allow for easy overriding, in particular for Guix/Nix users like me who would like to patch the code in a packaging step so all the various external deps don't have to be installed globally (but it would be useful to everyone else too and it's good practice anyway).

THIS IS A DRAFT PR. Just a start on how this might look and to get feedback.

Some considerations:

• rg/fd use is inconsistent with respect to remote files
• perform executable-find at the point of definition or at each point of use (where necessary)?
• doctor checks for executables depended-on by packages that themselves define a var for the executable should do executable-find on that var, not the raw command as a string

WDYT?