resolve: case sensitivity improvements

[benmosher]

• explore always ignoring case for CWD (fixes Problems if resolved path is absolute #310, All imports unresolved when using PowerShell #263)
• add a parameter to no-unresolved to ignore case
• make no-unresolved error message mention casing issue explicitly (i.e. 'file was resolved but case doesn't match')
• make other rules case-insensitive (i.e. named should not care that your case doesn't match the FS path)