Improve mono-repository support

[mjpitz]

In some cases, we assume the name of a library/application is the name of the repository in which the file resides. This works for many projects where you might use a repository per library or application but doesn't work as well for mono-repositories where files may be scattered across subdirectories. One simple way to handle this would be to pass along the file path in addition to the URL to the extraction logic.

The two cases that come to mind are:

• requirements.txt - in this case, we probably want to assume the library name to be the name of the directory the file resides in
• Gopkg.toml - in this case, it should be the inferred name (based on URL) plus the directory in the repo where it was found

Something nice about this issue is that the path is already available in the extractor process.. we simply need to update the internal interface and pass it through appropriately.