[SE-0226]: Remove test target dependencies from dependency resolution

[hartbit]

This is a fairly large Pull Request so here's a recap of the changes:

• Introduce a allRequiredTargets cached property on Manifest that calculates which targets are required to build the manifest's products. All other targets (automatically including test targets) are only used for that package's development, will never be importable from other packages and therefore don't participate in dependency resolution and are not required to build the package graph.
• Introduce a allRequiredDependencies cached property which returns the package dependencies required to build all manifest's products. All other dependencies are only necessary for development, will never be importable from other packages and therefore don't participate in dependency resolution and are not required to build the package graph.
• Introduce a isRoot property on PackageReference and Manifest to keep track of which packages/manifests are root, as this is necessary to calculate the previous two properties: indeed, all targets and all dependencies are visible from root packages.
• Use allRequiredDependencies during dependency resolution to avoid cloning dependencies which are not needed.
• Use allRequiredTargets and allRequiredDependencies during package graph building to omit non-required targets and dependencies from the graph. If not, the graph will fail for dependencies which were not cloned during dependency resolution.
• Write unit tests in ManifestTests to verify that allRequiredTargets and allRequiredDependencies return the expected targets and dependencies.
• Write "integration tests" in WorkspaceTests with a complex graph to make sure that dependency resolution and graph building works around different complicated cases.

This PR depends on #2423 being merged first.

[hartbit]

Question: Should we treat PackageGraphRootInput.dependencies like other dependencies, in that we only resolve target and dependencies which are reachable from products, or should they behave as root packages, in that we need all targets and dependencies from them?

The reason I ask that is that this PR makes the testCanResolveWithIncompatiblePins test fail because its setup with no root packages, and only a single root dependency. I can refactor it to have a root package with a single dependency, but I want to make sure I'm not hiding a regression.

[aciidgh]

Looks great in general but I'll do a deeper review later.

[hartbit]

@aciidb0mb3r I applied review comments, the largest of them being to move to a enum on PackageReference and Manifest to store the kind of package instead of two isLocal and isRoot properties. But it was worth it.

[hartbit]

I made improvements to this PR by using the fact that we now always have a package name (defaulting to a non-lowercased identity) to use that instead of identity in all user-facing messages.

[aciidgh]

LGTM! Thanks!!

[hartbit]

@swift-ci please smoke test

[hartbit]

@swift-ci please smoke test