Ask the planner what a pull request reaches; map a changed file onto modules in one place (hq ADR 0238)

touchedBy is now the only mapping of changed files onto modules — touched, added, and
read by no build — and reachOfMerge the planner's whole answer with the dependency walk.
The merge handler, the plan what-if, the merge gate's width and composition, and a pull
request's check all ask it, so planning and gating cannot disagree. The gate composes
the definitions of the modules a merge would rebuild or add, not every one in the tree,
and the check says the dependents a merge would build after them.

internal/link/build.go

@@ -111,6 +111,9 @@ type CheckRequest struct {
 	// the gate runs**, not the repository. With neither, only the repository's own merge-check.sh runs.
 	Modules []string `json:"modules,omitempty"`
 	New     []string `json:"new,omitempty"`
+	// Dependents are the modules a merge would build after Modules because they stand on them: the
+	// planner's dependency walk, said on the pull request.
+	Dependents []string `json:"dependents,omitempty"`
 	// Manifests are the touched modules' manifests in the change's tree, by path from its root: what the
 	// gate puts through `module check`.
 	Manifests []string `json:"manifests,omitempty"`