This is part of the work to create an apphost that bundles both hostfxr
and hostpolicy.  The main distinction between the static and shared
versions of hostfxr is that the static version contains a hostpolicy
resolver that references the hostpolicy symbols directly rather than
loading them from a DLL.

This change is part of the work to enable an apphost that bundles both
hostfxr and hostpolicy.  There's no distinction between hostpolicy
that's built as a shared library and as a static library: the shared
library is built by linking an empty object file with the static
library.

Provide a hostfxr_iface class, that abstracts how the hostfxr functions called
by the early stage in the hosting layer is resolved.

This provides two implementations of hostfxr_iface: one for the static
apphost, which bundles hostfxr and hostpolicy, and another for the
conventional apphost, which loads them dynamically on startup.

…host)

Add a test case

…loneApp)