Rename ssdtprojectsystem.targets to SQLProjectsSystem.targets

[Phantom-Sixth-Man]

Description

Adds support for the new SQLProjectsSystem.targets file used by SSMS and other IDEs, while maintaining backward compatibility with the existing ssdtprojectsystem.targets used by SSDT.

Background
The original ssdtprojectsystem.targets was specific to SSDT (SQL Server Data Tools). As SDK-style SQL projects are now supported by additional products like SSMS, a more generic SQLProjectsSystem.targets file is being introduced by those products.

Changes
Updated the import logic in Sdk.targets to:

Import ssdtprojectsystem.targets when it exists (SSDT installed) and SQLProjectsSystem.targets does not exist
Import SQLProjectsSystem.targets when it exists (SSMS/other IDEs installed) and ssdtprojectsystem.targets does not exist
These two targets files are mutually exclusive by design - only one will exist on a machine depending on which product is installed.

Backward Compatibility

• No breaking changes - existing SSDT installations continue to work as before
• The SDK automatically detects which targets file is available and uses the appropriate one
• No action required from the customers.

Code Changes

• Unit tests are added, if possible - N/A
• Existing tests are passing
• New or updated code follows the guidelines here
• Ensure .dacpac from changes to Microsoft.Build.Sql build process MUST be backwards compatible with older versions of SqlPackage
• Use proper logging for MSBuild tasks

[Matteo-T]

Looks good to me.

[Matteo-T]

LGTM