tests.yml: develop in-repo [sources] for monorepo ROOT (project: '.') on the LTS

[ChrisRackauckas-Claude]

Problem

tests.yml's "Develop in-repo [sources] path deps" step (and the helper checkout before it) is gated with:

if: "${{ inputs.buildpkg && inputs.project != '@.' && inputs.project != '.' }}"

So it runs for lib/* sublibraries (project: lib/<name>) but is skipped for a monorepo ROOT project passed as project: '.' — which is exactly what grouped-tests.yml passes for monorepo roots.

On Julia < 1.11 the [sources] table is not auto-resolved during build/test. A root project whose [deps] are pinned to in-repo siblings via [sources] therefore can't be built on the LTS. Concretely, on SciML/OptimalUncertaintyQuantification.jl (root [deps] = CanonicalMoments / DiscreteMeasures / OUQBase, each { path = "lib/<name>" } in root [sources]), julia-buildpkg on julia lts fails before any test runs:

ERROR: LoadError: expected package `CanonicalMoments [58d2c334]` to be registered

red-flagging tests / Core (julia lts) and tests / QA (julia lts).

Fix

Relax both gates to only exempt the default-environment sentinel '@.' (an ordinary single package, which by definition has no in-repo [sources]). '.' now develops its in-repo [sources] siblings just like a lib/* sublibrary.

develop_sources already handles the root correctly:

• collect_source_paths exempts the root project from the test-only-sources filter, so the root's own runtime [sources] are developed;
• it is a safe no-op when '.' declares no [sources], and on Julia >= 1.11 entirely.

No behavior change for @. callers (still skipped) or lib/* callers (still develop).

Local verification (Julia 1.10.11, against OptimalUncertaintyQuantification.jl)

• Bare Pkg.instantiate() on the repo root reproduces expected package CanonicalMoments ... to be registered.
• develop_sources(".") + Pkg.instantiate() resolves and precompiles the full stack (ModelingToolkit / Optimization / OUQBase + the three lib/* sources) cleanly — INSTANTIATE OK.

Rollout

After merge, re-tag v1 to the new master so @v1 callers (incl. OptimalUncertaintyQuantification.jl) pick up the fix.

Out of scope (separate root cause)

The Downgrade / Downgrade Sublibraries reds on the same monorepo are a julia-actions/julia-downgrade-compat bug (its create_merged_project leaves path-[sources] deps in the merged base [deps], so the resolver hits unknown package UUID), not a SciML/.github issue — reported separately.

Please ignore until reviewed by @ChrisRackauckas.