This commit fixes#6198.
When a project is removed from a solution using the `sln remove`
command, any projects in the solution with a project dependency (note:
this is different from a project reference) on the project should have
the project removed as a dependency.
The fix is to scan the projects in the solution and remove any
dependencies on the projects being removed. If the dependencies section
is empty after the remove, we skip serialization of the section like
Visual Studio does.
On Windows, `PathUtility.GetRelativePath` was not properly handling
paths that differed by case in the drive reference (e.g. "C:\" vs.
"c:\"). The fix was to add the missing case-insensitive comparison
argument.
Replaced uses of `PathUtility.GetRelativePath` with
`Path.GetRelativePath` where possible (requires 2.0.0+).
Additionally, `PathUtility.RemoveExtraPathSeparators` was not handling
paths with drive references on Windows. If the path contained a drive
reference, the separator between the drive reference and the first part
of the path was removed. This is due to `Path.Combine` not handling
this case, so an explicit concatenation of the separator was added.
This commit resolves issue #7699.
