Normalize project variables to on-disk case

### Steps to reproduce

Project file
```xml

```

Command line
```
s:\msbuild>dir TEst.proj
Volume in drive S is Source
Volume Serial Number is E68B-B87E

Directory of s:\msbuild

08/28/2018 01:45 PM 231 TEst.proj
1 File(s) 231 bytes
0 Dir(s) 146,367,778,816 bytes free

s:\msbuild>msbuild /nologo teST.PRoj
MSBuildProjectFullPath: s:\msbuild\teST.PRoj
MSBuildProjectName: teST

Build succeeded.
0 Warning(s)
0 Error(s)

Time Elapsed 00:00:00.47

s:\msbuild>msbuild /nologo TEst.proj
MSBuildProjectFullPath: s:\msbuild\TEst.proj
MSBuildProjectName: TEst

Build succeeded.
0 Warning(s)
0 Error(s)

Time Elapsed 00:00:00.48
```

### Expected behavior
`$(MSBuildProjectName)` and (more importantly) derived properties like `AssemblyName` in the default .NET Core SDK setup match the case on disk.

### Actual behavior
They match the case specified on the command line.

Start by reproducing the behavior with the project file shown in the issue and the three command-line casing variants. Trace where MSBuildProjectFullPath and MSBuildProjectName are initialized, then verify that they and derived properties such as AssemblyName preserve the on-disk case rather than the command-line case.

Written by the indexing model from the issue text.