Support running uninstalled in a tarball dir.

Running tests from the directory of an expanded tarball is a reasonable
thing to do, so support it by checking for PKG-INFO file if the pkg_resources
version grab fails.

Fixes bug 1103473

Change-Id: I2bf1fc39b070d3a2cdf6a3abe5e816c81d2916a0
Merge "Support running uninstalled in a tarball dir."