Allow absent inputs to use import contexts

lifecycle.go

@@ -83,8 +83,9 @@ type ImportDefaults struct {
 }
 
 // Options contains the application definition and per-load input selections.
-// Imports enables explicit import declarations; Overlay selects one fixed overlay;
-// Overrides contains only values explicitly supplied by an application CLI.
+// Imports enables traversal of declarations in a selected main document; an
+// absent source has no main declarations to traverse. Overlay selects one fixed
+// overlay. Overrides contain only values explicitly supplied by an application CLI.
 // Enabled source-version modes interpret Definition.Version.Path in each
 // document's local coordinates. Placement projects checks and main protection
 // through the input destination; item placement adds the source's item
@@ -109,9 +110,11 @@ type Options[T any] struct {
 }
 
 // FilesystemContext supplies roots for imports and overlays. With FS nil,
-// BaseDir and Root use absolute host paths. With FS set, they use slash-relative
-// paths in that namespace ("." denotes its root). File sources derive BaseDir
-// from the selected file; a native file's explicit Root limits its scope.
+// BaseDir and Root use absolute host paths. For NoSource, Root is required and
+// BaseDir may be omitted to use Root as the import base. With FS set, BaseDir
+// and Root use slash-relative paths in that namespace ("." denotes its root).
+// File sources derive BaseDir from the selected file; a native file's explicit
+// Root limits its scope.
 type FilesystemContext struct {
 	BaseDir string
 	Root    string