Add check for duplicate SQL build items

[zijchen]

The SDK's implicit globbing pattern (**/*.sql) may conflict with any explicit includes with metadata in the project file. Normally MSBuild is smart enough to resolve the duplicates, but the additional metadata makes it think that the items are different, which may result in a significant performance impact during the build process.

I'm adding a check similar to the CheckForDuplicateItems target that will fail the build if any explicit includes are already covered by the default globbing pattern. Project authors would either need to remove the explicit includes or disable the default globbing pattern by setting EnableDefaultSqlItems to false. Additional metadata should be attached using Update instead:

<Build Update="Table1.sql" Metadata="Value" />
  <Metadata>Value</Metadata>
</Build>