Extend Get-MSBuildProject to return the .user project.

Also fix the Get-ProjectFromName and Get-MSBuildProject parameters to
match the ProjectName property.
Rename Get-ProjectByName to Get-Projects and fix its semantics.

Get-Projects will return zero or more projects under the following
contitions:
a) Returns zero or all solution projects selected by wildcard if the
   ProjectName parameter is specified (ProjectName passed through to
   NuGet Get-Project)
b) Returns zero or all solution projects if the ProjectName parameter
   isn't specified (returns Get-Project -All)
Rewrite Update-Unity3DProjectReferences to use the MSBuild project.

Iterate over the project's MSBuild XML to find references to
Unity-managed DLLs.