Be smarter about PackagePath and Pack metadata values

See the documentation on adding package readme support, it states you need to add:

<PropertyGroup>
    ...
    <PackageReadmeFile>readme.md</PackageReadmeFile>
    ...
</PropertyGroup>

<ItemGroup>
    ...
    <None Include="docs\readme.md" Pack="true" PackagePath="\"/>
    ...
</ItemGroup>

Now, the PackagePath metadata is a packing-specific metadata item that is not used anywhere else (AFAIK). It seems quite obvious that if I specify a PackagePath metadata, it's because I want the item packed. Specifying also Pack=true seems entirely unnecessary and redundant.

So, the proposal is:

Whenever PackagePath is specified in any item (of the supported candidates), asume Pack=true for the item.

(this is what nugetizer does too, btw, it's more natural and less surprises).