edsp: document unique package identifiers

edsp: document unique package identifiers

Document that package identifiers must be unique (apt only uses the last
action for a given identifier) and that install requests do also imply
upgrades and downgrades (and thus removal of the old version). This is
to prevent that solvers express an upgrade or downgrade instruction as
two stanzas: a removal of the old version and an installation of the new
version. Instead, a single install stanza is sufficient to express
upgrade or downgrade requests.