Add Go module collection support

[dagger-codex]

Summary

Go workspaces get three selection dimensions — go-module, go-directory, go-test — with batched test execution:

$ dagger list go-test --go-directory=app/sub
TestSubDouble
TestSubZero

$ dagger check --go-test=TestSubDouble --go-test=TestSubZero
ok  example.com/app/sub  0.001s        # one go test -run '^(TestSubDouble|TestSubZero)$'

GoModule.testDirs exposes test directory discovery as a collection keyed by path; each directory exposes its tests as a collection keyed by name (go test -list). The collection-level run is a batch check that shadows the per-test run: selecting several tests compiles and runs them in a single go test invocation, and dimension filters push down so only selected directories enumerate.

Go.modules() returns a GoModules collection (keyed by workspace-relative module root path) instead of a plain list, making Go modules first-class artifacts: enumerable, selectable, and filterable through the standard collection surface.

$ dagger list go-module
./app
./lib

$ dagger call modules subset --keys ./app keys
./app

Authored with Dang's @keys/@get directives; the engine projects the public surface (keys, list, get(key), subset(keys)). Internal callers (lintAll, testAll, generateAll) iterate via the raw backing members, which stay visible in-module.

Rebased on main: the original marker-file plumbing was dropped in favor of main's skip-path configuration, which the collection simply carries to get.

Verification

Requires an engine with collection support (dagger/dagger#13299) — CI cannot pass until a release ships it. Validated against a dev engine built from that PR:

• e2e:module-introspection-check passes, including new assertions: get(key:) returns the requested module, subset(keys:) narrows exactly.
• Consumer workspace UX: dagger list go-module, dagger call modules keys / subset --keys ... keys work against this module.

Notes

• Test directory discovery (pre-existing go-includes behavior) does not surface test files at a module root; only subdirectories containing _test.go files become test directories.
• The check-selection flow (dagger check --go-test=…) additionally requires the plans-lite layer in [1.0-beta] modules-v2: add artifacts and collections dagger#13299.
• modules() discovers **/go.mod across the whole workspace; when the module source directory lives inside the workspace, its own embedded go.mods appear too. Pre-existing behavior, unchanged here.