Allow cmdlet import when default parameter set is AllParameterSets.

Cmdlets that define the DefaultParameterSetName to be
ParameterAttribute.AllParameterSets cannot be imported by Pash.
One example is the Get-Package cmdlet that is included with NuGet.

    [Cmdlet(VerbsCommon.Get, "Package",
        DefaultParameterSetName=ParameterAttribute.AllParameterSets)]
    public class GetPackageCommand : NuGetBaseCommand
    {

Importing such a cmdlet into the session state an 'An item with the
same key has already been added.' error displayed.

    var initialSessionState = InitialSessionState.CreateDefault();
    initialSessionState.ImportPSModule(moduleToImport);

   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at System.Management.Automation.CmdletInfo.GetParameterSetInfo(Type cmdletType) in Pash\Source\System.Management\Automation\CmdletInfo.cs:line 200
   at System.Management.Automation.CmdletInfo..ctor(String name, Type implementingType, String helpFile, PSSnapInInfo PSSnapin, ExecutionContext context) in Pash\Source\System.Management\Automation\CmdletInfo.cs:line 47
   at Pash.Implementation.CommandManager.<>c__DisplayClassb.<LoadCmdletsFromAssembly>b__8(Type type, CmdletAttribute cmdletAttribute) in Pash\Source\System.Management\Pash\Implementation\CommandManager.cs:line 183
   at System.Linq.Enumerable.<SelectManyIterator>d__31`3.MoveNext()
   at Pash.Implementation.CommandManager.LoadCmdletsFromAssembly(Assembly assembly, PSSnapInInfo snapinInfo) in Pash\Source\System.Management\Pash\Implementation\CommandManager.cs:line 184
   at Pash.Implementation.CommandManager.LoadCmdletsFromAssemblies(IEnumerable`1 assemblies) in Pash\Source\System.Management\Pash\Implementation\CommandManager.cs:line 195
   at Pash.Implementation.CommandManager.ImportModules(IEnumerable`1 modules) in Pash\Source\System.Management\Pash\Implementation\CommandManager.cs:line 205
   at Pash.Implementation.LocalRunspace.InitializeSession() in Pash\Source\System.Management\Pash\Implementation\LocalRunspace.cs:line 196
   at Pash.Implementation.LocalRunspace.Open() in Pash\Source\System.Management\Pash\Implementation\LocalRunspace.cs:line 147
Use default parameter set if it is the AllParameters set.

With a cmdlet specifying the default parameter set to be the
AllParameters set, when the parameter binding is ambiguous
, the default parameter set was not used if it is the
AllParameters set.

    [Cmdlet(VerbsDiagnostic.Test, "MyCmdlet",
        DefaultParameterSetName=ParameterAttribute.AllParameterSets)]
Merge pull request #201 from mrward/cmdlet-default-parameter-set-is-all-parameter-sets

Cmdlet default parameter set is all parameter sets