Unify file.emptyStub and file.imported === "implicit" as file.implicit.

Note that there was a typo in ResourceSlot#addStylesheet, so
file.emptyStub was never === true before.

tools/isobuild/compiler-plugin.js

@@ -664,7 +664,7 @@ class ResourceSlot {
         // If a compiler plugin calls addJavaScript with the same
         // sourcePath, that code should take precedence over this empty
         // stub, so this property marks the resource as disposable.
-        emtpyStub: true,
+        implicit: true,
         lazy: true,
       });
 

tools/isobuild/import-scanner.js

@@ -160,14 +160,14 @@ export default class ImportScanner {
     if (old) {
       // If the old file is just an empty stub, let the new file take
       // precedence over it.
-      if (old.emptyStub === true) {
+      if (old.implicit === true) {
         return this.absPathToOutputIndex[absPath] = file;
       }
 
       // If the new file is just an empty stub, pretend the _addFile
       // succeeded by returning the old file, so that we won't try to call
       // _combineFiles needlessly.
-      if (file.emptyStub === true) {
+      if (file.implicit === true) {
         return old;
       }
 
@@ -507,11 +507,11 @@ export default class ImportScanner {
       if (depFile) {
         // If the module was imported implicitly before, update to the
         // explicit version now.
-        if (depFile.imported === "implicit") {
+        if (depFile.implicit === true) {
           const file = this._readModule(absImportedPath);
           if (file) {
+            depFile.implicit = false;
             Object.assign(depFile, file);
-            depFile.imported = true;
           }
         }
 
@@ -837,13 +837,14 @@ export default class ImportScanner {
         servePath: relPkgJsonPath,
         hash: sha1(data),
         lazy: true,
+        imported: true,
         // Since _addPkgJsonToOutput is only ever called for package.json
         // files that are involved in resolving package directories, and
         // pkg is only a subset of the information in the actual
-        // package.json module, we mark it as being imported implicitly,
-        // so that the subset can be overridden by the actual module if
-        // this package.json file is imported explicitly elsewhere.
-        imported: "implicit",
+        // package.json module, we mark it as imported implicitly, so that
+        // the subset can be overridden by the actual module if this
+        // package.json file is imported explicitly elsewhere.
+        implicit: true,
       };
 
       this._addFile(pkgJsonPath, pkgFile);