Allow System and Local flags on all commands

This means that `PlacementLocation.userLocal` should be hardcoded almost nowhere, but now the `--system` and `--local` arguments work on any command that requires fetching, most notably `build`.

You can also see [here](https://github.com/rejectedsoftware/dub/pull/306/files#diff-9f8d774bfa99bc79eb6ab51899906213R87) I fixed a bug that caused `/` to be added to the beginning of absolute paths beginning with drive letters on windows.

Also, sorry about the whitespace diffs, my text editor removes all trailing whitespace on save, and sometimes I miss some of them when committing.