More descriptive logging

(from https://github.com/containers/BuildSourceImage/pull/11#issuecomment-529909863 by @vrothberg )
```
[SrcImg][INFO] calling sourcedriver_context_dir
[SrcImg][INFO] calling sourcedriver_extra_src_dir
[SrcImg][INFO] calling sourcedriver_rpm_dir
[SrcImg][INFO] calling sourcedriver_rpm_fetch
```

Starting with line 2, the log requires quite some understanding of the script. Maybe we can print some more descriptive messages on a higher level (as line 1 does)?

Start by locating the shell script that emits the [SrcImg][INFO] messages and review the sourcedriver calls shown in the issue. Replace the implementation-oriented messages with clearer higher-level descriptions, then run the available build or logging checks to confirm the output is understandable.

Scritto dal modello di indicizzazione a partire dal testo della issue.