Allow overriding dacpac filename for a package reference

Up until version 2.5.0 MSBuild.Sdk.SqlProj has assumed that the name of the .dacpac file in a PackageReference is the same as the ID of the package being referenced. Although this is the case in many scenarios, there are scenarios where a referenced package might contain multiple .dacpac's or the filenames might not be the same. Therefore we now support specifying a DacpacName attribute on a PackageReference that allows for overriding the name of the .dacpac being referenced from that package.

This is especially useful for referencing the new Microsoft.SqlServer.Dacpacs packages released by Microsoft, which contain the definitions of the master and msdb databases.