(Migrating from 1.x) 2.7.10 .deb packages installs in /var/lib/influxdb/.influxdbv2/, not /var/lib/influxdb/

[The docs](https://docs.influxdata.com/influxdb/v2/reference/internals/file-system-layout/?t=Linux#installed-as-a-package) specify that installing from .deb package installs in `/var/lib/influxdb/...`. However, on my server, things are in `/var/lib/influxdb/.influxdbv2`.

I installed by upgrading from InfluxDB v1. This might be due to me screwing something up while upgrading.

__Steps to reproduce:__

Install InfluxDB v1 .deb.
Run InfluxDB v1.

Install InfluxDB v2 .deb.
Migrate.

__Expected behaviour:__

Stuff in `/var/lib/influxdb/...` as shown in the docs.

__Actual behaviour:__

Stuff in `/var/lib/influxdb/.influxdbv2...` as if running standalone, sort of.

/etc/systemd/system/influxd.service
EnvironmentFile=-/etc/default/influxdb2

/etc/default/influxdb2
INFLUXD_CONFIG_PATH=/etc/influxdb/config.toml

/etc/influxdb/config.toml does not exist
/etc/influxdb/influxdb.conf still there

/var/lib/influxdb/.influxdbv2
configs
config.toml
engine/
influxd.bolt
influxd.sqlite

/var/lib/influxdb/.influxdbv2/config.toml
bolt-path = "/var/lib/influxdb/.influxdbv2/influxd.bolt"
engine-path = "/var/lib/influxdb/.influxdbv2/engine"
http-bind-address = "localhost:8086"
storage-series-id-set-cache-size = 100

__Environment info:__

Linux 5.10.0-26-cloud-amd64 x86_64

AFAIU, the systemd file points to the default config file location in /etc/influxdb/ but since that file is missing influxd defaults to standalone location.

It is not that bad, I mean it works, but it is kind of surprising. I had to search for my data. And since it is non-standard and works due to a default behaviour on a missing config file, I'm worried this might catch me some day in a later upgrade.

Start by comparing /etc/systemd/system/influxd.service, /etc/default/influxdb2, and the documented package layout, then reproduce the stated v1 .deb to v2 .deb migration. Done means the upgrade uses the documented /var/lib/influxdb path, or the documentation and configuration behavior clearly explain the .influxdbv2 location.

Written by the indexing model from the issue text.