A number of our integration tests set up sample projects that contain
`package.json` files. These files aren't actually needed to run the
tests, so this commit removes them to clean up the repository a bit.
Co-authored-by: Will Jones <will@sacharissa.co.uk>
It appears that a large number of tests were using references
to main fields which don't currently exist. The runtime didn't
produce an error because package.json was being ignored.
This is consistent with the behavior prior to the introduction of Read
steps. In order to avoid a breaking change we must do this check in the
engine itself, which causes a bit of a layering violation: because IDs
are marshaled as raw strings rather than PropertyValues, the engine must
check against the marshaled form of an unknown directly (i.e.
`plugin.UnknownStringValue`).