When IsPackable=true, default PackageId=AssemblyName

This matches the behavior in SDK pack and makes sense as a default when IsPackable is set, instead of requiring explicit PackageId,

Author: kzu

src/NuGetizer.Tasks/NuGetizer.Shared.targets

@@ -37,6 +37,9 @@ Copyright (c) .NET Foundation. All rights reserved.
   </PropertyGroup>
 
   <PropertyGroup Label="Hidden">
+    <!-- Matches the SDK Pack default when set to true. -->
+    <PackageId Condition="'$(IsPackable)' == 'true' and '$(PackageId)' == ''">$(AssemblyName)</PackageId>
+
     <IsPackable Condition="'$(IsPackable)' == '' and '$(PackageId)' != ''">true</IsPackable>
     <IsPackable Condition="'$(IsPackable)' == '' and '$(PackageId)' == ''">false</IsPackable>
     <!-- When the project is building a package, AssignPackagePath always assigns a PackagePath, regardless of the project PackageId -->

src/NuGetizer.Tests/given_an_empty_library.cs

@@ -10,6 +10,41 @@ public class given_an_empty_library
 
         public given_an_empty_library(ITestOutputHelper output) => this.output = output;
 
+        [Fact]
+        public void when_is_packable_true_then_package_id_defaults_to_assembly_name()
+        {
+            var result = Builder.BuildProject(@"
+<Project Sdk='Microsoft.NET.Sdk'>
+  <PropertyGroup>
+    <IsPackable>true</IsPackable>
+    <TargetFramework>netstandard2.0</TargetFramework>
+  </PropertyGroup>
+</Project>", output: output);
+
+            result.AssertSuccess(output);
+            Assert.Contains(result.Items, item => item.Matches(new
+            {
+                PackFolder = PackFolderKind.Metadata
+            }));
+        }
+
+        [Fact]
+        public void when_no_is_packable_and_no_package_id_then_defaults_to_non_packable()
+        {
+            var result = Builder.BuildProject(@"
+<Project Sdk='Microsoft.NET.Sdk'>
+  <PropertyGroup>
+    <TargetFramework>netstandard2.0</TargetFramework>
+  </PropertyGroup>
+</Project>", output: output);
+
+            result.AssertSuccess(output);
+            Assert.DoesNotContain(result.Items, item => item.Matches(new
+            {
+                PackFolder = PackFolderKind.Metadata
+            }));
+        }
+
         [Fact]
         public void when_getting_package_contents_then_includes_output_assembly()
         {