Import doesn't support Windows' paths, prevents use of absolute paths

[viper-admin]

Created by @mschwerhoff on 2016-09-27 07:25

For example, the following import statements fail:

import ".\foo.vpr"            // Can be rewritten to "./foo.vpr"
import "D:\Temp\test153.sil"  // Not sure if an absolute path can be rewritten

Since I don't see a good reason for not accepting paths in Windows format (the Java/Scala I/O libraries can handle Windows and Linux paths, AFAIK), we should support them.