For principal - subordinate plugin type relations where the
principal Python payload imports code from packages managed by a
subordinate, upgrades can be problematic.
This helper will allow a subordinate charm to inform its principal
about all implemented release - packages combinations ahead of time.
With this information in place the principal can do the upgrade in
one operation without risk of charm relation RPC type processing at
a critical moment.
Related-Bug: #1806111
Change-Id: I455501c1c2cdb53e62b533be95d2493bf1a5027a
ccb778f111