Show information to the user when a package is declared but no directory with that name exists

The program could print a warning message informing about the extra pacakge (or missing directory). For example:

```bash
$ cpm build
project.yaml:12: warning: directory $PACKAGE_NAME does not exist.
```

Start with the cpm build entry point and inspect how package declarations from project.yaml are resolved to directories. Reproduce the example with a package whose directory is absent, then ensure cpm build reports the package name and missing directory as a warning without preventing the intended build behavior.

Written by the indexing model from the issue text.