Trim memory usage of IReadOnlyPackage implementations.

These implementations are often backed by a Dictionary, and tend to live a long time after being loaded. Ensure TrimExcess is called on the backing dictionaries to reduce the long term memory usage. In some cases, we can also preallocate the dictionary size for efficiency.

OpenRA.Mods.Cnc/FileSystem/MegFile.cs

@@ -51,7 +51,7 @@ namespace OpenRA.Mods.Cnc.FileSystem
 		{
 			readonly Stream s;
 
-			readonly Dictionary<string, (uint Offset, int Length)> contents = new();
+			readonly Dictionary<string, (uint Offset, int Length)> contents;
 
 			public MegFile(Stream s, string filename)
 			{
@@ -84,6 +84,7 @@ namespace OpenRA.Mods.Cnc.FileSystem
 					throw new Exception("File name table in .meg file inconsistent");
 
 				// Now we load each file entry and associated info
+				contents = new Dictionary<string, (uint Offset, int Length)>((int)numFiles);
 				for (var i = 0; i < numFiles; i++)
 				{
 					// Ignore flags, crc, index
@@ -94,6 +95,8 @@ namespace OpenRA.Mods.Cnc.FileSystem
 					contents[filenames[nameIndex]] = (offset, (int)size);
 				}
 
+				contents.TrimExcess();
+
 				if (s.Position != headerSize)
 					throw new Exception("Expected to be at data start offset");
 			}