Bug

Fixes: https://github.com/NuGet/Client.Engineering/issues/3200
Fixes: https://github.com/NuGet/Client.Engineering/issues/3201

Description

Enables Visual Studio to migrate project.json projects without being directly tied to Restore via PackageSpec.

Creates a narrow PackageSpec type as a representation of a project.json migration candidate.
Copies relevant JSON parsing methods and other helper methods to read the 3 important properties:

• Library Dependencies
• Target Framework Dependencies
• Runtime Graph

Validation

Unit tests added covering ProjectJsonMigrationCandidatePackageSpecReaderTests which are similar to and adapted from tests in NuGet.ProjectModel.Test.JsonPackageSpecReaderTests.

Manually migrated a project.json project:

Tried multiple TargetFrameworks in a project.json to ensure an error is shown:

  "frameworks": {
    "net472": {},
    "netstandard2.0": {}
  },

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.