Create proper relative symbolic links

[manfred-brands]

Fixes #3730

Context

Symbolic link in the bin folder to obj/Assembly.dll were created just like that.

Changes Made

Converted relative paths to be relative to the target directory resulting in: ../obj/Assembly.dll
Modified NativeMethods.MakeSymbolicLink

Testing

Updated one unit test to test both absolute and relative paths. This failed before my changes, succeeds now.
As Path.GetRelativePath is not supported on netstandard2.0 and net472, I added a local version with appropriate unit tests.

Notes

[manfred-brands]

The azure pipeline doesn't give a proper line number for the failing MacOS test and I don't see any artefacts I can look at.
As I don't have access to a Mac I cannot run those test my self. Any suggestions on how to resolve this?

[Forgind]

Microsoft.Build.UnitTests.CopySymbolicLink_Tests.CopyToDestinationFolderWithSymbolicLinkCheck(useRelative: True)
System.IO.FileNotFoundException : Could not find file '/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/uiack5y3.3xu/2A333ED756AF4dc392E728D0F864A398/tmp5e9beda9b70849c6a8b53561d26ccb64.tmp'.
   at Interop.ThrowExceptionForIoErrno(ErrorInfo errorInfo, String path, Boolean isDirError)
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, UnixFileMode openPermissions, Int64& fileLength, UnixFileMode& filePermissions, Func`4 createOpenException)
   at System.IO.Strategies.OSFileStreamStrategy..ctor(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
   at System.IO.StreamReader.ValidateArgsAndOpenPath(String path, Encoding encoding, Int32 bufferSize)
   at System.IO.File.ReadAllText(String path, Encoding encoding)
   at Microsoft.Build.UnitTests.CopySymbolicLink_Tests.CopyToDestinationFolderWithSymbolicLinkCheck(Boolean useRelative) in /Users/runner/work/1/s/src/Tasks.UnitTests/Copy_Tests.cs:line 2441
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)\

[Forgind]

Did you write this method yourself? It looks correct to me, as far as I can tell, but it's substantially longer than an answer using URIs:
https://stackoverflow.com/questions/275689/how-to-get-relative-path-from-absolute-path

Might that work? My primary concern is that it'd be pretty easy to get this slightly wrong in an important way and have it fail. That is, I think it's right, but I'm not confident.

[manfred-brands]

As the comment in the method says, it is based upon .NET 7 Path.cs
The unit test cases also come from .NET7.

I updated the code to using the Uri.MakeRelativeUri method as used in the article, but many of the unit test cases failed. Even after adding a trailing slash, still many tests failed. Seeing all the comments on the article, I'm not the only one. Uri also have a far more restricted character set than files.

I therefore think the original code is appropriate.

Why does msbuild need a build target for net472 and netstandard20? Do you update msbuild for older SDKs?

[JanKrivanek]

LGTM

[JanKrivanek]

@ladipro this PR will need RequiresSymbolicLinksTheory - can you please add it (reusing the RequiresSymbolicLinksFact)

[manfred-brands]

@ladipro this PR will need RequiresSymbolicLinksTheory - can you please add it (reusing the RequiresSymbolicLinksFact)

Done.

[ladipro]

@ladipro this PR will need RequiresSymbolicLinksTheory - can you please add it (reusing the RequiresSymbolicLinksFact)

Done.

Thank you for taking care of it! We may want to move the code from the constructor to a helper method so it's not duplicated in RequiresSymbolicLinksTheory and RequiresSymbolicLinksFact but that's not a blocking issue.

[ladipro]

And apologies for missing Jan's mention.