Initial ecosystem implementation for .NET/C#.

[mitchell-as]

CP-980 Implement C# ecosystem installer in State Tool

The dotnet command, provided by the .NET runtime/SDK, functions like npm for JavaScript. It replaces nuget's functionality. (This means it is possible to use dotnet to pull down packages from the nuget online package registry.) We cannot use nuget in Linux because of it's dependency on Mono, which is a large framework we should absolutely not depend on.

However, dotnet requires a project to operate in, so we cannot install *.nupkg artifacts directly like we can with npm. Instead, we unpack *.nupkg artifacts into a directory structure dotnet recognizes. Then users can build their own projects using our packages.

The order of operations is:

• Checkout a .NET Platform project.
• Install a .NET package.
• Start a shell for the project, which sets the $NUGET_PACKAGES environment variable that points to our virtual environment's package listing.
• Go to your .NET project and run dotnet restore, which will read your .NET project's project file and verify you have all the required dependencies (which should be in our virtual environment).
• Build and run the .NET project.

After the state install command, I turned off my network to confirm that dotnet can detect and use packages in our virtual environment.

It's a bit unfortunate that dotnet restore needs the --source $NUGET_PACKAGES argument, but oh well. It's certainly possible that people building against a local package listing have to do something similar to prevent a reachout to the nuget registry, so it may not be so bad.

[mitchell-as]

The default .NET package registry lives in the user's ~/.nuget/packages/ directory. We can override this with the $NUGET_PACKAGES environment variable, and we'll point it to this directory: /usr/nupkg/.

[Naatan]

Looks ok to me, one minor nit.

[Naatan]

Suggested change

	if file.Name() == "runtime.json" {
	err = injectEnvVar(file.AbsolutePath(), "NUGET_PACKAGES", "${INSTALLDIR}/"+e.nupkgDir)
	if err != nil {
	return nil, errs.Wrap(err, "Unable to add NUGET_PACKAGES to runtime.json")
	}
	continue
	}
	if !strings.HasSuffix(file.Name(), ".nupkg") {
	continue
	}
	if !strings.HasSuffix(file.Name(), ".nupkg") {
	continue
	}
	if file.Name() == "runtime.json" {
	err = injectEnvVar(file.AbsolutePath(), "NUGET_PACKAGES", "${INSTALLDIR}/"+e.nupkgDir)
	if err != nil {
	return nil, errs.Wrap(err, "Unable to add NUGET_PACKAGES to runtime.json")
	}
	continue
	}

Reduce chance for failures.

[mitchell-as]

Actually, that will prevent a runtime.json file from ever being processed. If there wasn't a !, then I'd agree with your proposed changes.