Disallow package in compat when its absent in deps

[vdayanand]

For eg: https://github.com/JuliaRegistries/General/blob/master/G/GalacticOptim/Compat.toml adds a restriction on Zygote for GalacticOptim v0.1.0 even though its not a dependency.

[ericphanson]

I think this is often OK; for example, you can use compat to restrict the versions of test-only dependencies, which can be important for tests to pass! And a package might happen to know it is incompatible with another package even if they don't depend on each other.