[trivial] Make rt.minfo import in object_.d function-local.

The 2.065 frontend trips over this when building the druntime
tests in shared library mode with LDC (forward reference to the
Object base class of TypeInfo when accessing the latter from
within rt.util.container). No idea why this happens, or why it
doesn't with DMD, but this fixes it and it also is a small
improvement on its own regardless.
Merge pull request #857 from klickverbot/minfo-import

[trivial] Make rt.minfo import in object_.d function-local.