Banning `ImmutableSet` outright is too much to do all at once - this starts
the process by banning `ImmutableMap#entrySet` - one of the more common ways
that `ImmutableSet`s come up. It then starts to remove calls to
`ImmutableMap#entrySet` by changing declarations from `ImmutableMap` to `Map`.
Unfortunately this process is like pulling on a long, windy string and one
declaration change requires another which requires 5 more which in turn
require another few. So this change is rather large.
As such, to keep the changes manageable they only remove `ImmutableMap` from
the signatures that are needed for `entrySet` and make little effort to stop
using `ImmutableMap` internally. Removing the usages of `ImmutableMap`
complicates immutability guarantees and will be done separately.
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