[FEATURE] Revert thin amboso change from 1.9

[jgabaut]

Ever since amboso turned into a thin script sourcing amboso_fn.sh (so >=1.9), I've been wondering if it was actually a good choice.

Having the script be a single file would allow for easier portability (avoiding the need to precisely locate the symbols to source), but it may come at a cost.

For now, the idea was that a globally-installed anvil would check the amboso-dir directory to source the local symbols.

• This would be scrapped and the project compatibility would be checked by an explicit key in the stego.lock file (currently, it would be the anvil.version key).
• This would break the existing link between the project-provided anvil and a system one