Enable autoSearchPaths to match pyright's CLI

[dreddnafious]

Fixes #221.

scip-python builds its own CommandLineOptions and never sets autoSearchPaths, so
ensureDefaultExtraPaths() never adds ./src. pyright's CLI sets it unconditionally:

// pyright-internal/src/pyright.ts
// Always enable autoSearchPaths when using the command line.
options.configSettings.autoSearchPaths = true;

Since pyright-scip replaces that entry point, the setting is lost, and src-layout projects
index with definitions under src.pkg.mod while imports resolve to pkg.mod. The two namespaces
never join, so cross-package references are dropped. The run exits 0 and the index is just smaller.

Verification

Minimal project, no [tool.pyright] section:

demo/
├── pyproject.toml       # [project] name = "demo", version = "0.1.0"
├── src/demo/__init__.py
├── src/demo/core.py     # def handler() -> int: return 1
└── tests/test_core.py   # from demo.core import handler

Before — the reference to handler is absent entirely:

src/demo/core.py
    def: scip-python python demo 0.1.0 `src.demo.core`/handler().
tests/test_core.py
    ref: scip-python python demo 0.1.0 `demo.core`/__init__:

After (equivalently, with extraPaths = ["src"] set manually on 0.6.6):

src/demo/core.py
    def: scip-python python demo 0.1.0 `demo.core`/handler().
tests/test_core.py
    ref: scip-python python demo 0.1.0 `demo.core`/handler().
    ref: scip-python python demo 0.1.0 `demo.core`/handler().

For comparison, pyright itself resolves this tree with no configuration at all — same directory,
no [tool.pyright]:

$ npx pyright@latest --outputjson
{ "summary": { "filesAnalyzed": 3, "errorCount": 0, ... } }

So pyright and scip-python currently disagree about the same project.

Safety

ensureDefaultExtraPaths applies autoSearchPaths and explicit extraPaths independently, and
the auto-detected path is only added when ./src exists and is not itself a package
(src/__init__.py absent). Projects that already configure extraPaths in pyrightconfig.json
or [tool.pyright] are unaffected; projects with a src package directory are unaffected.

Note on tests

I did not add a snapshot fixture. A snapshots/input/src_layout/ case (a src/-rooted package
plus an importer outside src/) would cover this well, but the expected output has to come from
npm run update-snapshots against a build, and I would rather not hand-write generated output I
can't verify. Happy to add it if you'd like — just say the word and I'll build and generate it
properly.

Environment

• verified against scip branch and @sourcegraph/scip-python 0.6.6 (current npm latest)
• compared against pyright 1.1.411
• Node v24.13.0, Linux