Move undocumented environment variable names to a common location

Author: tamasvajk

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/DependencyManager.cs

@@ -111,7 +111,7 @@ public DependencyManager(string srcDir, IDependencyOptions options, ILogger logg
                 logger.LogInfo($"Unresolved reference {r.Key} in project {r.Value}");
             }
 
-            var webViewExtractionOption = Environment.GetEnvironmentVariable("CODEQL_EXTRACTOR_CSHARP_BUILDLESS_EXTRACT_WEB_VIEWS");
+            var webViewExtractionOption = Environment.GetEnvironmentVariable(EnvironmentVariableNames.WebViewGeneration);
             if (webViewExtractionOption == null ||
                 bool.TryParse(webViewExtractionOption, out var shouldExtractWebViews) &&
                 shouldExtractWebViews)

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/EnvironmentVariableNames.cs

@@ -0,0 +1,8 @@
+namespace Semmle.Extraction.CSharp.DependencyFetching
+{
+    internal class EnvironmentVariableNames
+    {
+        public const string WebViewGeneration = "CODEQL_EXTRACTOR_CSHARP_BUILDLESS_EXTRACT_WEB_VIEWS";
+        public const string MonoPath = "CODEQL_EXTRACTOR_CSHARP_BUILDLESS_MONO_PATH";
+    }
+}

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/Runtime.cs

@@ -80,7 +80,7 @@ private IEnumerable<string> DesktopRuntimes
 
                 string? monoDir = null;
 
-                var monoPathEnv = Environment.GetEnvironmentVariable("CODEQL_EXTRACTOR_CSHARP_BUILDLESS_MONO_PATH");
+                var monoPathEnv = Environment.GetEnvironmentVariable(EnvironmentVariableNames.MonoPath);
                 if (monoPathEnv is not null)
                 {
                     if (Directory.Exists(monoPathEnv))
@@ -89,7 +89,7 @@ private IEnumerable<string> DesktopRuntimes
                     }
                     else
                     {
-                        logger.LogError($"The directory specified in CODEQL_EXTRACTOR_CSHARP_BUILDLESS_MONO_PATH does not exist: {monoPathEnv}");
+                        logger.LogError($"The directory specified in {EnvironmentVariableNames.MonoPath} does not exist: {monoPathEnv}");
                     }
                 }
                 else