Perform basic validations for each runtime during `func pack`

[im-samz]

Description

Child item of: #3881

Description

Background
func pack is positioned to support .NET, Python, Node, and Custom Handler projects.

For each, it'd be good to perform some basic validations to catch any issues at build-time.

UX
In the terminal output, perhaps we can add a section, "Validating project". Then, in real time, we can list the validations we are performing and whether it "PASSED" or "FAILED". We can discuss if this belongs more in a --verbose scenario, but I do think it would be nice to convey the checks we are running.

If any validation fails, we should log the validation's title with an actionable message to self-resolve.

• Title is to help with SEO when looking for best practice resolutions. We could also create a Learn doc to map validations with resolutions.
• Actionable message is to help with self-resolution

Extra

• Let me know how I can help with actionable error messages.
• In the future, we can think more about what to do in the --no-build scenarios beyond the basics we put here.

Validations

.NET

Title	Validation	Notes
Validate Folder Structure	The deployment payload should match the output of a dotnet publish command, though without the enclosing parent folder. The zip archive should be made from the following files: .azurefunctions/, extensions.json, functions.metadata, host.json, worker.config.json, any project executables (a console app), and other supporting files and directories peered to that executable.	Check this once dotnet publish has been run. If --no-build is specified, ensure that the directory structure is still as follows.

Python

Title	Validation	Notes
Validate OS Compatibility	Check if the customer is running “func pack” from a Windows machine.	Throw a warning, noting the risk of native dependencies and recommend using --build-native-deps if they have docker installed.
Validate Folder Structure	Check for the existence of function_app.py, requirements.txt, .python_packages/, and host.json	Check in the --no-build scenario as well; .python_packages/ should not be empty either; throw an error if failed
Validate Python Programming Model	Check that the customer isn’t trying to use both V1 and V2 programming models in the same app (function_app.py and function.json should not exist)	Check in the --no-build scenario as well; throw an error if failed

Node.js

Title	Validation	Notes
Validate Folder Structure	Check for the existence of “package.json” and “host.json” at the root.	throw an error if failed

PowerShell

Title	Validation	Notes
Validate Folder Structure	Check for the existence of “host.json"; at least one folder contains “functions.json”	throw an error if failed

Customer Handlers
check host.json and confirm that the custom handler executable exists. If the file is missing, we can issue a warning, since the customer may add the configuration by other methods as well.