Please make abstreet package-friendly

Data can be pre-downloaded and installed into a read-only location by the package, and can also be downloaded in the runtime into the user's home directory.

Currently the application doesn't seem to understand that the location is read-only and breaks with "something wrong" message: https://github.com/a-b-street/abstreet/issues/803

When no data is present - the application just fails with 'No such file or directory': https://github.com/a-b-street/abstreet/issues/802

It should always offer the user to download data into the home directory instead of into the installed location. And this should be the only option when the installed location is read-only.

Look at the data loading logic, likely in a module like `data` or `importer`. The issue references two other issues (#802, #803) that show specific error messages. Start by finding where the application checks for data files and where it writes downloaded data. The fix involves detecting a read-only installation directory and defaulting to a user-writable location like the home directory. Test by running the application with data missing or in a read-only location.

Escrito por el modelo de indexación a partir del texto del issue.