I originally introduced `package_canonical` into the API schema because,
at the time, we were thinking to "pin" the version of the MCP server to
_one_ of the referenced source code packages.
We are no longer doing this (we are maintaining a separate notion of
`version` unique to the MCP server), so I don't believe
`package_canonical` is needed anymore. Does anyone see otherwise?
This change removes all mentions in READMEs and usages in code.
