Allow conversion of projects with unrecognized imports

[mjrousos]

This updates try-convert's response to unrecognized imports. Rather than failing the conversion, the conversion proceeds and the unrecognized imports are left unchanged.

If the imported props or targets file is not one recognized as an allowable import, the user is given a message warning that the import may not work with the converted project file.

Fixes #370

[mjrousos]

@cartermp, here's the imports update we talked about a few days ago. Turns out it was easier than I expected to get the desired behavior because the project root element still has the old imports and there's an explicit converter step to remove them - so I just updated that.

[ardalis]

Would love to see a release of this soon! Thanks!

[cartermp]

Thanks!