`--build-summary` does not create a report for local packages

[edsko]

Passing build-summary when installing something from Hackage creates a build report, but when I run cabal install with --build-summary on a package in a local directory it does not. (This is related to #735, but not quite the same, I think.)

Note that although --remote-build-reporting indeed makes no sense for packages that are installed from a local directory, that is not true for --build-summary (indeed, as a developer, I might want to upload a build report for one my packages directly to Hackage, if it cannot be built on the Hackage server -- maybe because of missing C libraries, for instance.)