Fix external source files reference with "code" pattern in project.json

[ChengTian]

• Reference external source files with forward slashes on Windows works now
• Reference external source files with backslashes on Mono works now

parent #559

It also doesn't work if you use backslashes in code pattern on Mono to reference source files outside project root. This PR fix the problem at the same time.

[davidfowl]

After this change goes in and passes, change the sources throughout the asp net vnext repositories to use / instead of \. We'll get tons of coverage this way.

[ChengTian]

Sounds great! I can create an issue for it. For such an issue that affects all repositories, should I create it in Home repository?

[davidfowl]

Nope, just do it!