Move IDataView into Microsoft.ML namespace

[eerhardt]

We've received feedback that having IDataView in the Microsoft.Data namespace (which was done in #2220) is confusing to users. Traditionally, the System.Data namespace has been populated with "database" specific technologies. Microsoft.Data isn't all that different from System.Data, and one could argue they are basically the "same thing", with the System.Data technologies being inside of the BCL and/or netstandard definition while Microsoft.Data being built outside of the BCL/netstandard definition. There are already "database" specific APIs in Microsoft.Data, for example https://docs.microsoft.com/en-us/dotnet/api/microsoft.data.sqlite.

System.Data even already has a DataView class, and IDataView and DataView are drastically different.

To solve this confusion, we should rename the Microsoft.Data.DataView NuGet package to Microsoft.ML.DataView and move the Microsoft.Data.DataView namespace under Microsoft.ML. The NuGet package will still be separated from the algorithms and transforms in the larger Microsoft.ML NuGet package. So the value of #1860 can still be realized. The separate package will just be branded ML instead of Data.

/cc @TomFinley @shauheen @markusweimer @glebuk

[eerhardt]

With solving this, I think it would be safe to reconcile one more issue: the separation of the base DataViewTypes (like Int32, Single, etc) and the Vector and Key types that happened with #2220. Since all the types are under Microsoft.ML, it doesn't make sense for these types to be separated anymore. We can bring VectorType and KeyType into the same assembly as the base DataViewTypes.

Note: this means that VBuffer would also need to come with, since VectorType has a hard reference to VBuffer.

[markusweimer]

@shauheen Shall we make this part of Project 13? This seems like the sort of thing we must solve before 1.0.

[shauheen]

Should this proceed as proposed it certainly will be Project 13.