This allows us to share the function between `initializeInstance` and
`makeSolutionBuilderHost`. `makeSolutionBuilderHost` is called as a part
of `buildSolutionReferences`.  `buildSolutionReferences` is called before
`initializeInstance` where the custom transformers are initialized.

This allows us to pass on the custom transformer while the project
references are built.

`options.configFilePath` is always a string, there's no reason we need `toString()` too.

In a previous commit, I removed `.toString()` without validating what
the Typescript compiler would say. This change does a runtime check
that the project variable is an actual string.