Build fails when using a path using colon character

### Steps to reproduce

Originally reported against Visual Studio for Mac - https://devdiv.visualstudio.com/DevDiv/_workitems/edit/807811

Wondering if it is valid to use a directory with ':' on the Mac and for msbuild to support this. Using dotnet sdk as an example:

1. mkdir test:abc
2. cd test:abc
3. dotnet new console
4. dotnet build

### Expected behavior

Build should work?

### Actual behavior

Build fails:

```
dotnet build
Microsoft (R) Build Engine version 15.9.20+g88f5fadfbe for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

Restore completed in 30.61 ms for ~/Projects/Tests/test:abc/test:abc.csproj.
/usr/local/share/dotnet/sdk/2.1.503/15.0/Microsoft.Common.props(66,3): error MSB4019: The imported project "~/Projects/Tests/test:abc/obj/test:abc.csproj.*.props" was not found. Confirm that the path in the declaration is correct, and that the file exists on disk. [~/Tests/test:abc/test:abc.csproj]

Build FAILED.

/usr/local/share/dotnet/sdk/2.1.503/15.0/Microsoft.Common.props(66,3): error MSB4019: The imported project "~/Projects/Tests/test:abc/obj/test:abc.csproj.*.props" was not found. Confirm that the path in the declaration is correct, and that the file exists on disk. [/Users/matt/Projects/Tests/test:abc/test:abc.csproj]
0 Warning(s)
1 Error(s)

```

The obj directory contains:

project.assets.json
test:abc.csproj.nuget.cache
test:abc.csproj.nuget.g.props
test:abc.csproj.nuget.g.targets

### Environment data
`msbuild /version` output:

Microsoft (R) Build Engine version 15.9.20+g88f5fadfbe for .NET Core

OS info:

Mac OS 10.13.6