circular dependency loops among packages should be flagged as errors

I just did a test and got a surprising result -- these two packages give no error, even back to M2 1.12:
```m2
newPackage ( "Foo",
PackageImports => { "Bar" }
)

-----------------------------------------------------------------------------

newPackage ( "Bar",
PackageImports => { "Foo" }
)
```

Will fix.

_Originally posted by @DanGrayson in https://github.com/Macaulay2/M2/issues/1950#issuecomment-1011247096_