Fixed parsing of the imported modman file from nested folder

Hello, I was not able to install the https://github.com/fooman/speedster module and did some debug of the `doImport` method.

First fix makes possible to read the file from import instructions.

The second, large fix converts the relative paths `../../../../../app/code/community/Fooman/Speedster/` to `app/code/community/Fooman/Speedster/` with `realpath` command.

The second fix is more like dirty hack and I feel that this logic should be moved to `Modman_Command_Link_Line` class or elsewhere, but I don't have the `sModuleDirectory` available inside that class.