chore: Specify IsPackable=false on Directory.Build.props, explicitly true for target packages.

.gitignore

@@ -100,7 +100,19 @@ publish
 *.Publish.xml
 
 # NuGet Packages Directory
-packages
+*.nupkg
+# NuGet Symbol Packages
+*.snupkg
+# The packages folder can be ignored because of Package Restore
+# packages # upm pacakge will use Packages
+# **/[Pp]ackages/*
+# except build/, which is used as an MSBuild target.
+# !**/[Pp]ackages/build/
+# Uncomment if necessary however generally it will be regenerated when needed
+#!**/[Pp]ackages/repositories.config
+# NuGet v3's project.json files produces more ignorable files
+*.nuget.props
+*.nuget.targets
 
 # Windows Azure Build Output
 csx