error: unable to find PyOxidizer config file

It would be nice to detect the toml file in an existing project and note that it needs to be converted to the new .bzl format.

I'm also getting an error after conversion
```console
Updating git repository `https://github.com/indygreg/PyOxidizer.git`
error: failed to load source for a dependency on `cpython`

Caused by:
Unable to update https://github.com/indygreg/PyOxidizer.git?rev=6f325f307dbef8f2fb970b1980f8f1206a2786d6

Caused by:
revspec '6f325f307dbef8f2fb970b1980f8f1206a2786d6' not found; class=Reference (4); code=NotFound (-3)
```

updating Cargo.toml to use `35a70354ba4e0e24e632f0427a7ebf0cb1c56344` seems to do the trick, but then ran into a series of other problems with my old Cargo.toml, and I abandoned the project upgrade path, as it easier to start a new project and copy the few added bits.

At some point a `pyoxidizer upgrade` will be needed, but atm it might be a good idea to detect old versions and recommend that users create new projects for 0.6 instead of trying to get a 0.5 project upgraded.

Start by reproducing the upgrade path from an older PyOxidizer project using its existing TOML and Cargo.toml files, then inspect the `pyoxidizer upgrade` entry point. Check how old project versions are identified and how configuration-file errors are reported. Done means old projects receive a clear recommendation to create a new 0.6 project instead of proceeding with an unreliable upgrade.

Written by the indexing model from the issue text.