Detect when position is not in known source

[nicolasstucki]

If the position is outside of the known contents of the source we do not return any source.

Fixes #14785

[michelou]

@nicolasstucki On line 6 of test file tastyinspector/i14785.scala I would write split(File.pathSeparatorChar) instead of split(':').

[som-snytt]

@nicolasstucki and on line 7, Windows wants the leading slash dropped, apparently.

Caused by: java.nio.file.InvalidPathException: Illegal char <:> at index 2: /C:/actions-runner2/_work/dotty/dotty/compiler/../out/bootstrap/scala3-compiler-bootstrapped/scala-3.1.3-RC1-bin-SNAPSHOT-nonbootstrapped/scala3-compiler_3-3.1.3-RC1-bin-SNAPSHOT.jar

as verified locally

scala> Paths.get(p)
java.nio.file.InvalidPathException: Illegal char <:> at index 2: /C:/cygwin64/ho
me/andre/.sdkman/candidates/scala/2.13.4/lib/scala-compiler.jar
  at java.base/sun.nio.fs.WindowsPathParser.normalize(WindowsPathParser.java:182
)
  at java.base/sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:153)
  at java.base/sun.nio.fs.WindowsPathParser.parse(WindowsPathParser.java:77)
  at java.base/sun.nio.fs.WindowsPath.parse(WindowsPath.java:92)
  at java.base/sun.nio.fs.WindowsFileSystem.getPath(WindowsFileSystem.java:229)
  at java.base/java.nio.file.Path.of(Path.java:147)
  at java.base/java.nio.file.Paths.get(Paths.java:69)
  ... 32 elided

scala> Paths.get(p.drop(1))
val res26: java.nio.file.Path = C:\cygwin64\home\andre\.sdkman\candidates\scala\
2.13.4\lib\scala-compiler.jar

I'm glad I left cygwin installed.