add a test for #4278 and adjustments for enabling warning as errors

[smoothdeveloper]

Giving it a try on CI, locally, for the added test, I face some MSbuild nonsense:

C:\dev\src\github\fsprojects\Paket\integrationtests\scenarios\i004278-net90-windows-fails\before\before.sln.metaproj : error MSB4126: The specified solution configuration "Debug|x64" is invalid. Please specify a valid solution configuration using the Configuration and Platform properties (e.g. MSBuild.exe Solution.sln /p:Configuration=Debug /p:Platform="Any CPU") or leave those properties blank to use the default solution configuration. [C:\dev\src\github\fsprojects\Paket\integrationtests\scenarios\i004278-net90-windows-fails\before\before.sln]

I did minor tweaks to have warning as errors, the change in src/Paket.Core/Dependencies/DependenciesFileParser.fs is worth some attention, but the rest is straight forward.

[smoothdeveloper]

https://ci.appveyor.com/project/paket/paket/builds/51128543/job/968us0x0k371xuyt/tests

#4278 Support net 9 windows platformpaket.integrationtests.dll9 sec
Std OUT:
\u001b[37;2mC:\projects\paket\integrationtests\scenarios\i004278-net90-windows-fails\temp already exists.\u001b[0m
C:\projects\paket\integrationtests\scenarios\i004278-net90-windows-fails\temp> paket install
\u001b[32mC:\projects\paket\bin\net461\paket.exe install\u001b[0m
Performance:

The new test seems to be in place on CI, and no other change compared to the master branch in the CI output.