Use depends on package syntax

[isomorphisms]

Change the API package manifest from depends = network to depends on network and teach the package parser to accept on in the separator position.

The manifest itself is the integration probe: a compiler that cannot parse the new form cannot build/install idris2api.ipkg.

One grammar caveat remains before this should merge: the minimal parser change is currently in the shared equals rule, so it also accepts on for other package fields. If we want on to be legal only after depends, the final form should split out a dedicated dependency separator in Idris.Package rather than broaden the shared separator.