Fix -Wconf:src Windows path conversion

[mbland]

Updates the -Wconf:src filter to avoid using java.nio.file.Path.toURI in order to fix Windows source path conversions.

Path.toURI prepends the current working directory to Windows-like paths unconditionally, and converts backslashes in such paths to %5C escape sequences This can cause -Wconf:src filters that work on non-Windows platforms to fail on Windows.

For example, before this change, the Path.toURI conversion in the SourcePattern case from MessageFilter.matches() produced:

original: Optional[C:\foo\bar\myfile.scala]
resolved: /Users/mbland/src/scala/scala3/C:%5Cfoo%5Cbar%5Cmyfile.scala

After this change, it produces the following, which still prepends the current working directory, but properly converts path separators to /:

original: Optional[C:\foo\bar\myfile.scala]
resolved: /Users/mbland/src/scala/scala3/C:/foo/bar/myfile.scala

This change is based on scala/scala#11192, and also adapts some test cases from that pull request to validate symlink and normalized path handling. This change also extracts the diagnosticWarning helper method to reduce duplication between new and existing test cases.

Fixes #24771. Review by @lrytz.

[lrytz]

After this change, it produces the following, which still prepends the current working directory, but properly converts path separators to /:

original: Optional[C:\foo\bar\myfile.scala]
resolved: /Users/mbland/src/scala/scala3/C:/foo/bar/myfile.scala

After blowing the dust off my Windows VM, I can confirm this is platform dependent.

On Windows:

scala> java.io.File("C:\\foo\\..\\bar\\f.txt").toPath.toAbsolutePath.normalize
val res0: java.nio.file.Path = C:\bar\f.txt

scala> java.io.File("C:\\foo\\..\\bar\\f.txt").toPath.toAbsolutePath.toUri.normalize.getRawPath
val res1: String = /C:/bar/f.txt

On macOS:

scala> java.io.File("C:\\foo\\..\\bar\\f.txt").toPath.toAbsolutePath.normalize
val res0: java.nio.file.Path = /Users/luc/code/scala/scala13/sandbox/C:\foo\..\bar\f.txt

scala> java.io.File("C:\\foo\\..\\bar\\f.txt").toPath.toAbsolutePath.toUri.normalize.getRawPath
val res1: String = /Users/luc/code/scala/scala13/sandbox/C:%5Cfoo%5C..%5Cbar%5Cf.txt

So arguably the current implementation using toUri is also OK..?