Implement name an signature conflict check across extension containers with the same receiver type (#77747)

Implements the following rule from the spec:
"Within a given enclosing static class, the set of extension member declarations with the same receiver type (modulo identity conversion and type parameter name substitution) are treated as a single declaration space similar to the members within a class or struct declaration, and are subject to the same rules about uniqueness."

Author: AlekseyTs

src/Compilers/CSharp/Portable/Symbols/ConversionSignatureComparer.cs

@@ -9,7 +9,7 @@
 
 namespace Microsoft.CodeAnalysis.CSharp.Symbols
 {
-    internal sealed class ConversionSignatureComparer : IEqualityComparer<SourceUserDefinedConversionSymbol>
+    internal sealed class ConversionSignatureComparer : IEqualityComparer<MethodSymbol>
     {
         private static readonly ConversionSignatureComparer s_comparer = new ConversionSignatureComparer();
         public static ConversionSignatureComparer Comparer
@@ -24,7 +24,7 @@ private ConversionSignatureComparer()
         {
         }
 
-        public bool Equals(SourceUserDefinedConversionSymbol member1, SourceUserDefinedConversionSymbol member2)
+        public bool Equals(MethodSymbol member1, MethodSymbol member2)
         {
             if (ReferenceEquals(member1, member2))
             {
@@ -53,7 +53,7 @@ public bool Equals(SourceUserDefinedConversionSymbol member1, SourceUserDefinedC
                 && (member1.Name == WellKnownMemberNames.ImplicitConversionName || member2.Name == WellKnownMemberNames.ImplicitConversionName || member1.Name == member2.Name);
         }
 
-        public int GetHashCode(SourceUserDefinedConversionSymbol member)
+        public int GetHashCode(MethodSymbol member)
         {
             if ((object)member == null)
             {