pkg_install: strip tree only once

Strip directory tree only once. This is preparation for multipart
pkg_install support.
pkg_install: don't overwrite filelists when called multiple times

Don't overwrite filelists when called multiple times. This is preparation
for multipart pkg_install support.
pkg_install: implement pkg_install_part

Implement pkg_install_part, which will allow to divide a package into
a multiple "subparts". Each part's files are listed in a separate file.
pkg_install: separate debuginfo from file lists

Separate debuginfo from file lists.