Expose logic for finding local cache path

Needed so that FileSystemProvider can override the default behavior, since it doesn't use the same cache logic as other providers.

src/LibraryManager/Providers/BaseProvider.cs

@@ -375,7 +375,7 @@ protected async Task<ILibraryOperationResult> WriteToFilesAsync(ILibraryInstalla
         /// Gets the expected local path for a file from the file cache
         /// </summary>
         /// <returns></returns>
-        private string GetCachedFileLocalPath(ILibraryInstallationState state, string sourceFile)
+        protected virtual string GetCachedFileLocalPath(ILibraryInstallationState state, string sourceFile)
         {
             return Path.Combine(CacheFolder, state.Name, state.Version, sourceFile.Trim('/'));
         }

src/LibraryManager/Providers/FileSystem/FileSystemProvider.cs

@@ -206,5 +206,28 @@ protected override string GetDownloadUrl(ILibraryInstallationState state, string
         {
             throw new NotSupportedException();
         }
+
+        protected override string GetCachedFileLocalPath(ILibraryInstallationState state, string sourceFile)
+        {
+            // FileSystemProvider pulls files directly, no caching.  So here we need to build a full
+            // path or URI to the file.
+
+            // For HTTP files, the state.Name is the full URL to a single file
+            if (FileHelpers.IsHttpUri(state.Name))
+            {
+                return state.Name;
+            }
+
+            // For other filesystem libraries, the state.Name may be a either a file or folder
+            // TODO: abstract file system
+            if (File.Exists(state.Name))
+            {
+                return state.Name;
+            }
+
+            // as a fallback, assume state.Name is a directory.  If this path doesn't exist, it will
+            // be handled elsewhere.
+            return Path.Combine(state.Name, sourceFile);
+        }
     }
 }