Implement import of v1 objects.inv files

[bskinn]

No idea how practically useful this would be, but it's a feature.

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A Google search on 2018-08-15 for "Sphinx inventory version 1" yielded 432 hits, of which only two pages worth were deemed by Google to be unique enough to be worth displaying.

Closing, since (i) the demand for this is likely to be vanishingly small, and (ii) the v1 files are already in plaintext, and thus should be easy to read.