feat: require packageManager in package.json (#8017)
### Description
With 2.0 we will now be requiring a `packageManager` field in
`package.json` as this is a best practice and it helps us behave in a
deterministic manner.
The actual code change is very straightforward as we remove our package
manager inference code and return an error if reading package manager
from `package.json` fails.
Most of the PR is updating tests.
### Testing Instructions
Updated unit tests
