`bundle lock --print --update` can take a long time to fetch sources and
resolve the lock file.

Before, `--print` caused output to be completely silenced, so nothing
was printed at all until the resolved lock file is finally emitted to
stdout.

With this change, `--print` now prints progress to stderr. E.g.:

```
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
```

This provides a better user experience, especially when
`lock --print --update` takes several seconds or more.

The lock file is still printed to stdout, so tools consuming the lock
file on stdout will not be affected.

https://github.com/rubygems/rubygems/commit/6719baa700