Installer: Restrict entry filter to files from the BFS volume

We generally want to skip the contents of the packagefs volumes (save
for the shine-through directories). That makes Installer usable again.
In what direction we want to develop it (e.g. integrate some PM support,
so that a subset of packages can be selected) needs further discussion.

Installer: Remove "/boot/var" directory check

The PM Haiku Installer doesn't need support installing ancient Haiku
versions.

Installer: Move path string constants to new InstallerDefs.h/cpp

Installer: Restrict entry filter to files from the BFS volume

We generally want to skip the contents of the packagefs volumes (save
for the shine-through directories). That makes Installer usable again.
In what direction we want to develop it (e.g. integrate some PM support,
so that a subset of packages can be selected) needs further discussion.

Installer: Remove "/boot/var" directory check

The PM Haiku Installer doesn't need support installing ancient Haiku
versions.

Installer: Move path string constants to new InstallerDefs.h/cpp