Fix apiviewgo download paths

[chlowell]

This fixes a couple problems causing apiviewgo to fail when run by APIView:

• os.UserHomeDir returns an unwriteable path on APIView's production VMs. Fixed this by instead (trying to) write to os.TempDir (%TMPDIR% is writeable on those VMs)
• Code for computing a package's name relative to its module (e.g. "azcore/log") assumed the module name appears exactly once in the module's disk path and that the disk path includes the module path verbatim. Neither assumption holds true for downloaded code, and in general disk and module paths are unrelated. Fixed this by making module and disk path separate parameters of NewPkg() (this is much simpler than having NewPkg() figure out the difference)