Dashboard variable: throw an error when query variable contains other variables

__Proposal:__
This is a follow up on https://stackoverflow.com/questions/67038206/how-to-get-a-query-variable-on-influxdb-2-0-dashboard
A dashboard will will silently fail or display no data when a query variable contains reference to another variable inside its definition.

For example, this query variable definition will be accepted:
```
from(bucket: "pool")
|> range(start: v.timeRangeStart, stop: v.timeRangeStop)
|> filter(fn: (r) => r._measurement == "minerstat")
|> keep(columns: ["account"])
|> distinct(column: "account")
|> keep(columns: ["_value"])
```

__Current behavior:__
A cell using the corresponding variable will fail to display anything. The dropdown menu of the variable display "no value".

__Desired behavior:__
Detect the inner variable reference and throw an error at variable creation time or variable use time.

__Alternatives considered:__
Initially I saved the query data in a CSV and created a CSV variable.
Then I replaced `start: v.timeRangeStart, stop: v.timeRangeStop` with `start: -1d` to have the desired behavior.

__Use case:__
This was an easy newbie mistake that can generates a lot of frustration and hours to understand the root cause.

No file, test, or entry point is named. Trace dashboard query-variable creation and use paths first, then identify where references to other variables can be detected; done means the invalid definition or use produces an explicit error instead of an empty dropdown or silent failure.

Written by the indexing model from the issue text.