Full support of LSL

[WIP][Spec] Full support of the Lightweight Solution Load

Problem

NuGet client in VS is not fully functional in LSL mode. Currently only restore operation is supported. The main problem is NuGetProject and other componenets are not aware of LSL mode as all information from VS are retrieved via EnvDTE or CPS API.

Solution

NuGet VS client needs to support LSL mode without breaking it.

Development activity is tracked in NuGet/Home#5058.

List of NuGet operations per project model

Operation	packages.config	project.json	PackageRef	.NET Core
Manage packages (solution)	Supported	Supported	Supported
Manage packages (project)	Loads on right-click	Loads on right-click	Loads on right-click
Install package (PM)	Loads project
Uninstall package (PM)	Loads project
Update package (PM)	Loads project
Execute init.ps1 (PMC)	Supported
Add-BindingRedirect	Loads project
Find-Package	Supported
Get-Package	Supported
Get-Project	Loads project
Install-Package	Loads project
Open-PackagePage	Supported
Sync-Package	Supported
Tab expansion (PMC)	Supported
Uninstall-Package	Loads project
Update-Package	Loads project
IVsPackageInstallerServices	Supported	Supported	Supported

Shared Infrastructure

A project adapter will be introduced to abstract away communication with EnvDTE and CPS API where needed. The adapter implementation will be able to detect LSL mode and switch between data sources and APIs. It will force load project if no equivalent data is available in LSL DB.

packages.config

In LSL mode it facilitates `PackagesConfigReader' to retrieve package references. Works correctly in LSL mode for majority of read-only operations. For install, uninstall, update operations will force load project as it needs to update assembly references in the project.

project.json

Utilizes JsonPackageSpecReader to get a PackageSpec for the project.

PackageRef

Relies on IDeferredProjectWorkspaceService to get all necessary project data.

.NET Core (Roslyn project system)

Does not support LSL. No action needed.