Rewrite dependencies tree

[drewnoakes]

Fixes #8940 Dependencies tree refactor
Fixes #3642 Dependency node does not handle configuration switches
Fixes #6183 Use Dataflow and snapshots for dependencies node data processing
Fixes #2618 Dependencies node fails to handle custom target frameworks
Fixes #2525 Remove all concepts of "target framework" and that configurations are driven from them
Fixes #2795 Remove concept of "short name" in TargetFramework
Fixes #5044 Don't show dependency warning icons while waiting for design-time build
Fixes #5519 DependenciesSnapshotProvider is marked with [ProjectAutoLoad] with a dynamic capability
Fixes #5045 Provide feedback that dependencies node is populating
Fixes #8998 When changing a package version, update rather than replace the tree node
Fixes #2524 Dependency node doesn't handle Multi-targeting with a multi portable frameworks
Fixes #2617 TargetFramework fails to differentiate by NuGet profile name
Fixes #6203 Multiple equivalent target framework short names cause exception

This rewrite brings the dependencies tree in line with the current best practices for modelling project data. Specifically:

• Using standard dataflow blocks for merging data across configurations (replacing the previous AggregateCrossTargetProjectContext and related types).
• Using "slices" to simplify data handling and correctly support switching the active configuration.
• Making a snapshot of dependencies available at various levels, simplifying certain behaviours (previously we'd only have deltas, so components needed to track their own state independently).

The dependencies tree now populates much sooner, and no longer shows yellow triangles during project load for projects that are already restored, which greatly improves perceived performance. Fewer tree updates also improve actual performance.

A simpler representation of dependencies is now made. Instead of IDependencyModel which had many properties, we now have IDependency with only a few. If a dependency exposes a browse object, it may also implement IDependencyWithBrowseObject which adds a few extra properties. If we expose additional capabilities in future, we can add more interfaces to support that, which providers may opt in to.

The rewrite introduces the distinction between "configured" and "unconfigured" dependencies. For example, all dependencies coming from MSBuild are configured, while for a TypeScript/JavaScript project NPM packages are unconfigured (they have no concept of MSBuild configurations). Unconfigured dependencies are displayed at the top level, while configured dependencies are displayed beneath a node reflecting their configuration (when multi-targeting).

We also introduce an abstraction for MSBuild properties that come from project/joint rule sources. There is no common representation for dependencies in MSBuild, so a "factory" abstraction is introduced to take care of mapping between items and their metadata to a common representation.

A new DependencyGroupType type was added to model information common to all dependencies of a given type.

Other miscellaneous changes in this rewrite:

• Remove all explicit handling of target frameworks from the code. Everything is now performed in terms of "slices" which include target frameworks and potentially other implicit configuration dimensions.
• Tree building now occurs directly on IDependency, removing the need for the additional IDependencyViewModel object and the copying associated with that.
• Improve tree update logic to avoid collapsing expanded nodes during updates.
• Move all "legacy" extensibility code into its own folder, and implement in terms of the new APIs in LegacyDependencySubscriber.
• Added static class KnownProjectImageMonikers which caches ProjectImageMoniker instances, avoiding the locking and work done within ToProjectSystemType() on each use of an image moniker at run time.
• Consolidation of namespaces. Originally the dependencies tree was in the VS layer. In 16.x much of the code was moved to the host agnostic layer, however namespaces could not be updated. This rewrite moves us towards addressing that, where possible, by replacing the previous.
• Update documentation on the dependencies node, and add a bunch more API documentation.

Microsoft Reviewers: Open in CodeFlow

[tmeschter]

I went through the change, but it is so substantive that there is a limit to how useful a review can be. :-) I did have a few minor comments.