Makes Python plugin to output

for standalone Python files now, instead of nothing as now.
Before the change, no task context was created for the standalone file
due to `VariableName::RelativeFile` lookup considered as an error.
Now, Zed continues and constructs whatever possible context instead.
That `pytest` task seems odd, as the logic fixed here needs a relative
path (hence, a worktree) to consider unit tests.
We do not have variables at the moment the associated tasks are queried
for:
14920ab910/crates/languages/src/python.rs (L359-L363)14920ab910/crates/languages/src/python.rs (L417-L446)
so we cannot filter this the same way the PR does.
Maybe, we can use a `VariableName::RelativeFile` instead of
`VariableName::File` there?
Release Notes:
- Show tasks from Python plugin for standalone files