[darc/maestro++] Improve darc gather-drop's dry run

The dry run ability today stops just short of downloading files. Location information is verified, but not much else. Dry run should be improved so:
- [ ] It checks the file exists and is accessible (this also means dealing with credentials)
- [ ] It checks downloading the file would not overwrite existing files (unless --overwrite is specified)
- [ ] We can baseline away dependencies we expect not to be able to find builds for (e.g. pinned version of xdt)