VS Insertion tool to honor package entries in private *.config files.

Currently all packages inserted into VS are in one huge default.config file. With the recent change to split this to manageable team specific config entries to avoid Merge conflict hell for insertions, we'd want to ensure that the insertion tool also looks at these other config files for package updates.

[This wiki](https://dev.azure.com/devdiv/DevDiv/_wiki/wikis/DevDiv.wiki/4272/Splitting-default.config) states the changes made to split the default.config. Here's the format that lists additional config files we'd want to look at for package version updates:
```

```
where each config has a content schema as follows:
```

```

Start by locating the VS insertion tool and reviewing the linked Splitting-default.config wiki entry. Trace how default.config package versions are read, then support the additionalPackageConfigs entries and their referenced package files; done means package updates are found in both the default and listed private config files.

Written by the indexing model from the issue text.