Implement --exclude-dependencies to make --exclude exclude a package's dependencies

[kemzeb]

Describe the feature

Related discussions are here and here.

Currently, --exclude does its job of excluding packages from the dependency tree. This is fine, but when there are times when we also want to remove their dependencies from the output the current implementation doesn't make this possible.

This issue proposes an --exclude-dependencies flag that will have pipdeptree take this additional step of removing dependencies from the output.

In the future major versions, it may make sense to just make this the default behavior of --exclude. But to avoid a breaking change, it should just be a flag for now.