[openlilylib] Rename 'snippet-' to 'module-'

[Urs Liska]

Hi all openlilylib users,

when updating information on a few snippets (to test the documentation 
generator) I think I realized something.

Currently we call the files we have ready for inclusion "snippets", 
which is partially true but also a 'historic' thing.

But I think such a file can provide one of the following:

- one distinct object (e.g. function, shorthand, engraver, stylesheet)
  that is available after inclusion
- some functionality that is processed immediately on inclusion
- a number of objects (e.g. functions) that are made available
  through inclusion.

I think "snippet" doesn't match that list, and I suggest to rename it, 
which would also make the library even more different from the LSR.

My suggestion is to think of the files as 'modules'. I think this is the 
most consistent naming, also consistent with the use in programming 
languages.
Say I "include the module 'git-commands'", then I have the contained 
commands availabe from within the LilyPond input file. If I "include a 
module 'stylesheets/henle/concert/piano'" the included stylesheets are 
immediately applied.

This would imply changing the header field names from "snippet-title" to 
"module-title" etc., meaning some more work when restructuring the 
library. But I think it's worth the work.

What do you think?

Urs