C#: Separate all classes to dedicated files in CIL extractor

csharp/extractor/Semmle.Extraction.CIL/PDB/ISourceFile.cs

@@ -0,0 +1,20 @@
+namespace Semmle.Extraction.PDB
+{
+    /// <summary>
+    /// A source file reference in a PDB file.
+    /// </summary>
+    public interface ISourceFile
+    {
+        string Path { get; }
+
+        /// <summary>
+        /// The contents of the file.
+        /// This property is needed in case the contents
+        /// of the file are embedded in the PDB instead of being on the filesystem.
+        ///
+        /// null if the contents are unavailable.
+        /// E.g. if the PDB file exists but the corresponding source files are missing.
+        /// </summary>
+        string? Contents { get; }
+    }
+}