Make `depends on` package syntax work

[isomorphisms]

The intended Idriç package spelling is:

depends on network

It is not currently working as package syntax.

PR #13 is the attempted syntax change, but it remains a draft and its current parser patch broadens the shared separator. Issue #17 records the separate constraint that on must be accepted only after depends, not after unrelated package fields.

Acceptance:

• depends on ... parses and works in a real package manifest;
• existing package syntax remains valid;
• on is confined to dependency declarations;
• the manifest integration probe succeeds.