Huge Tool refactor

Copy of Changelog entry

⚠ There are major changes, please look at v0.7 migration guide.

Added

• Template pack for dotnet new: dotnet new -i CodeGeneration.Roslyn.Templates
• Various samples in samples folder
• GitHub Actions CI
• Support for plugin dependencies! 🎉 (#156).
• Plugins (generators) are now easier to build using CodeGeneration.Roslyn.Plugin.Sdk MSBuildSdk package (#113).
• Plugins are now easier to distribute using CodeGeneration.Roslyn.PluginMetapackage.Sdk MSBuildSdk package, see
  Readme section (#205).

Changed

• Readme demo is now simpler, and suggests usage of Templates package, and Sdks.
• .NET Core SDK version bumped to 3.1.100 (#178).
• Attributes package now targets net20;net40 in addition to netstandard1.0 (#178).
• Tool now has RollForward=Major policy to allow it to run on newer runtimes than 2.x,
  e.g. .NET Core SDK v3.x only should suffice for most usage scenarios (#178).
• MSBuild ItemGroup used for registration of plugin paths changed to CodeGenerationRoslynPlugin
  (was GeneratorAssemblySearchPaths). A warning for using old one is introduced (CGR1002). (#156)
  • ItemGroup now should contain full path to generator dll (previously it was a containing folder path)
  • Old behavior has a compat-plug for now and the paths are searched for any dll, and those found are added to new ItemGroup.
  • When using P2P generator (same solution), a consuming project needs to add an attribute OutputItemType="CodeGenerationRoslynPlugin" to the ProjectReference of the generator project. See v0.7 migration guide.
• dotnet-codegen package is now CodeGeneration.Roslyn.Tool and is built very differently;
  also it includes build assets from BuildTime package (#198).

Removed

• CodeGeneration.Roslyn.BuildTime package is now merged into CodeGeneration.Roslyn.Tool
  (which is now the only package required to be referenced by generator consumers, aside from generators themselves) (#198).