Update asmdef files when project references are added/removed

[citizenmatt]

This PR will:

• Automatically update a .asmdef file when a reference to another .asmdef based project is added or removed. Fixes Reference assembly action should update asmdef files #852 (and RIDER-48660)
• Remove references from .asmdef file if a reference is removed from a .csproj file. This means the .asmdef file will be updated when e.g. the Remove Unused References refactoring is used. Fixes Remove unused references #1994
• Project references are added as GUID based references, unless existing name based references are being used.
• Asmdef references are correctly handled when modifying or adding/removing .Player projects. However, the reference change is not automatically propagated to the Player/non-player project until Unity regenerates the project files.

This PR does not currently handle reference modifications other than asmdef based changes. References to custom assemblies, or plugin assemblies (DLLs that are assets) are currently ignored. It might be best to notify the user that changes like this will be lost the next time Unity regenerates the project.

Similarly, it does not update the "autoReferenced" property. This is true by default, and means all predefined assemblies (e.g. Assembly-CSharp) reference all .asmdef projects. If a reference to a .asmdef project is added to a predefined project, it would imply this flag is set to false. We can update this in this circumstance. However, it is unclear what we should do if a .asmdef project is removed from a predefined project. Should we automatically set the flag to false, or prompt first? We would also have to update any other references in predefined projects to be consistent.