Duplicate dependency field analyzer (#5463)

* Duplicate dependency field analyzer

Detects cases of duplicate [Dependency] fields in a type. We apparently have 27 of these across RT + SS14.

* Fix duplicate dependencies in Robust

Robust.Analyzers.Tests/DuplicateDependencyAnalyzerTest.cs

@@ -0,0 +1,63 @@
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.CSharp.Testing;
+using Microsoft.CodeAnalysis.Testing;
+using Microsoft.CodeAnalysis.Testing.Verifiers;
+using NUnit.Framework;
+using VerifyCS =
+    Microsoft.CodeAnalysis.CSharp.Testing.NUnit.AnalyzerVerifier<Robust.Analyzers.DuplicateDependencyAnalyzer>;
+
+namespace Robust.Analyzers.Tests;
+
+[Parallelizable(ParallelScope.All | ParallelScope.Fixtures)]
+[TestFixture]
+[TestOf(typeof(DuplicateDependencyAnalyzer))]
+public sealed class DuplicateDependencyAnalyzerTest
+{
+    private static Task Verifier(string code, params DiagnosticResult[] expected)
+    {
+        var test = new CSharpAnalyzerTest<DuplicateDependencyAnalyzer, NUnitVerifier>()
+        {
+            TestState =
+            {
+                Sources = { code }
+            },
+        };
+
+        TestHelper.AddEmbeddedSources(
+            test.TestState,
+            "Robust.Shared.IoC.DependencyAttribute.cs"
+        );
+
+        // ExpectedDiagnostics cannot be set, so we need to AddRange here...
+        test.TestState.ExpectedDiagnostics.AddRange(expected);
+
+        return test.RunAsync();
+    }
+
+    [Test]
+    public async Task Test()
+    {
+        const string code = """
+            using Robust.Shared.IoC;
+
+            public sealed class Foo
+            {
+                [Dependency]
+                private object? Field;
+
+                [Dependency]
+                private object? Field2;
+
+                [Dependency]
+                private string? DifferentField;
+
+                private string? NonDependency1;
+                private string? NonDependency2;
+            }
+            """;
+
+        await Verifier(code,
+            // /0/Test0.cs(9,21): warning RA0032: Another [Dependency] field of type 'object?' already exists in this type as field 'Field'
+            VerifyCS.Diagnostic().WithSpan(9, 21, 9, 27).WithArguments("object?", "Field"));
+    }
+}