`apt-get -s upgrade` calculates the upgrade differently depending on whether InRelease files are present in the cache

[Xiao Wan]

# Background
I have an in-house package manager that downloads packages from mirrors in parallel and maintains a versioned local repo. It has worked well since 2019. However, during a recent migration from 18.04 to 22.04 I encountered a strange bug: the install step keeps failing because the download step has missed the following packages: xserver-common xserver-xorg-core and xserver-xorg-legacy. It turns out that `apt-get -s`, which the download step uses to figure out the packages that need to be downloaded, produces different results depending on whether InRelease files are present in the cache (as specified in `Dir::State::Lists`). I suspect Phased-Update-Percentage is a related factor.

# How to reproduce
1. Launch a virtual machine with `xubuntu-22.04.2-desktop-amd64.iso` (sha256: c7072859113399bef0e680a08c987c360f41642d8089ebb8928689066a9c9759)

2. Download `minimal.tar.xz` at https://github.com/xwcal/ubuntu-apt-bug/blob/main/minimal.tar.xz?raw=true (sha256: ff7c38c8db2f05813dd641da504182180bf81d72cca9306a0898684c6b839b65) and run `tar -xf minimal.tar.xz` under `/some/dir` to extract the index files. They were taken from a real scenario last week.

3. Replace the content of /etc/apt/sources.list with the following three lines:
```
deb file:/home/apt/repo jammy main universe multiverse restricted
deb file:/home/apt/repo jammy-security main universe multiverse restricted
deb file:/home/apt/repo jammy-updates main universe multiverse restricted
```

4. run `apt-get -s -o Dir::State::Lists=/some/dir/lists2 upgrade` and observe the output -- you should hopefully get:
```
The following packages have been kept back:
  linux-generic-hwe-22.04 linux-headers-generic-hwe-22.04 linux-image-generic-hwe-22.04
...
123 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.
```

5. `rm /some/dir/*InRelease*` and then run `apt-get -s -o Dir::State::Lists=/some/dir/lists2 upgrade` again -- you should hopefully get:
```
The following packages have been kept back:
  linux-generic-hwe-22.04 linux-headers-generic-hwe-22.04 linux-image-generic-hwe-22.04 xserver-common xserver-xorg-core xserver-xorg-legacy
...
120 upgraded, 0 newly installed, 0 to remove and 6 not upgraded.
```

# Desired behavior
`apt-get -s` should produce consistent results regardless of whether InRelease files are present in the cache.

[Julian Andres Klode (juliank)]

Deleting InRelease files from the repository is not supported. The release file is mandatory for phased update support to correctly ignore phasing for security updates, as it identifies security repositories based on the Archive: field in the InRelease file.

[Xiao Wan (xwcal)]

@juliank Thanks for confirming my guess regarding Phased-Update-Percentage. However, I don't think I have gained any better understanding of the role Release/InRelease files play in the upgrade calculation (I assume by "Archive:" you meant "Suite:"?).

Doesn't the file naming scheme (managed by apt-get itself) already provide enough information as to whether a package is a security update? Why not just go through the `*security*Packages` files in the cache to exclude those listed packages from phased update?

If Release/InRelease files are required to correctly calculate an upgrade (if, say, `apt-get update` does not yet verify the files it fetches), shouldn't `apt-get upgrade` display an error message and fail conspicuously instead of silently producing an incorrect calculation?