Fix #16659 - Unwanted node modules copying

[nul800sebastiaan]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes #16659

Description

As noted in #16659, everything from App_Plugins is copied into the web output. This is unneeded and in the case of the node_modules directory unwanted behavior, these files are sources and not output that Umbracco needs to serve packages/extensions.

• The old behavior has led to many of the "path too long" errors we've been seeing while building extensions.
• It also makes the regular dotnet build step needlessly long on the first run as it needs to copy over loads of small files.
• This also affects dotnet publish commands, the whole node_modules folder is included in the published output that people will try to put on a server, again leading to very slow copying times AND path too long errors

The new behavior avoids all these problems.

Reproduction

• Set up a v15 RC2 site
• Add these files in the directory where the csproj file is App_Plugins.zip
• Run a dotnet build for this csproj file and note that in bin\Debug\net9.0\App_Plugins\vanilla-extension a node_modules folder appears
• Completely delete the bin\Debug folder
• Check out this PR locally and build the NuGet packages with the command: dotnet pack --configuration Release --output .\release\
  • Note: run this command in the directory where umbraco.sln is located, you will see a release directory appearing which will contain the NuGet outputs for Umbraco
• Copy the path of the release directory produced above, for example: E:\Dev\Umbraco-CMS15\release
• In your existing v15 RC2 site, where the csproj file is, run the following command: dotnet add package Umbraco.Cms --prerelease --source E:\Dev\Umbraco-CMS15\release (the --source path is the one you copied in the last step).
• Now run another dotnet build and note that the node_modules directory is NOT in the bin\Debug\net9.0\App_Plugins\vanilla-extension directory

Here's a video of the steps I took to verify the before and after behavior:

fix16659.mp4

[nul800sebastiaan]

This should be cherry-picked to v14 when merged as well.

Note: I ran into this because I was following the new Extending the backoffice training. I had problems building because of the infamous "path too long" error. I already have long path names enabled on my machine, so that does not always fix the problem.

[nikolajlauridsen]

Cherry-picked into the release 😄