Provide better feedback when projects fail to resolve

Currently, if you declare projectScope mappings for project paths that don't exist, there is no error logged. The resulting module will have incorrect module.xml that is missing the 'scope' value for the `` elements. Without this, the module will fail to run.

We should validate the project paths exist as part of the plugin, and throw or log an error if they aren't found.

Start at the plugin handling projectScope mappings and trace how project paths are resolved into module.xml. Reproduce the case with a nonexistent project path, then verify that the plugin reports the missing path and prevents module.xml from omitting the jar scope value.

Written by the indexing model from the issue text.