Allow apt to run if no dpkg/status file exists

Not having a dpkg/status file used to be a hard error which from a boostrap perspective is suspect as in the beginning, there is no status so you would need to touch it into existence.

We make a difference between factual non-existence and inaccessibility to catch mistakes in which the file is not readable for some reason, the testcase test-bug-254770-segfault-if-cache-not-buildable is an example of this.

The opportunity is used to (re)move an old hack from our testing framework which ensured that there always was a dpkg/status file causing a dpkg package to mysteriously appear in some testcases, so that we test this change with most of our testcases now as we will usually start with no status file present.