Prepare semver (@latest) releases in CI

Now that we track package versions in source, `@latest` builds should
be fully reproducible for a given commit. We can prepare the packages in
CI and store them as artifacts, the same way we do for `@next` and
`@experimental`.

Eventually this can replace the interactive script that we currently
use to swap out the version numbers.

The other nice thing about this approach is that we can run tests in CI
to verify that the packages are releasable, instead of waiting until
right before publish.

I named the output directory `oss-stable-semver`, to distinguish from
the `@next` prereleases that are located at `oss-stable`. I don't love
this naming. I'd prefer to use the name of the corresponding npm dist
tag. I'll do that in a follow-up, though, since the `oss-stable` name is
referenced in a handful of places.

Current naming (after this PR):

- `oss-experimental` → `@experimental`
- `oss-stable` → `@next`
- `oss-stable-semver` → `@latest`

Proposed naming (not yet implemented, requires more work):

- `oss-experimental` → `@experimental`
- `oss-next` → `@next`
- `oss-latest` → `@latest`

scripts/rollup/build-all-release-channels.js

@@ -98,9 +98,29 @@ function processStable(buildDir) {
     updatePackageVersions(
       buildDir + '/node_modules',
       versionsMap,
-      defaultVersionIfNotFound
+      defaultVersionIfNotFound,
+      true
     );
     fs.renameSync(buildDir + '/node_modules', buildDir + '/oss-stable');
+
+    // Identical to `oss-stable` but with real, semver versions. This is what
+    // will get published to @latest.
+    spawnSync('cp', [
+      '-r',
+      buildDir + '/oss-stable',
+      buildDir + '/oss-stable-semver',
+    ]);
+    const semverVersionsMap = new Map();
+    for (const moduleName in stablePackages) {
+      const version = stablePackages[moduleName];
+      semverVersionsMap.set(moduleName, version);
+    }
+    updatePackageVersions(
+      buildDir + '/oss-stable-semver',
+      semverVersionsMap,
+      defaultVersionIfNotFound,
+      false
+    );
   }
 
   if (fs.existsSync(buildDir + '/facebook-www')) {
@@ -131,7 +151,8 @@ function processExperimental(buildDir, version) {
     updatePackageVersions(
       buildDir + '/node_modules',
       versionsMap,
-      defaultVersionIfNotFound
+      defaultVersionIfNotFound,
+      true
     );
     fs.renameSync(buildDir + '/node_modules', buildDir + '/oss-experimental');
   }
@@ -179,7 +200,8 @@ function crossDeviceRenameSync(source, destination) {
 function updatePackageVersions(
   modulesDir,
   versionsMap,
-  defaultVersionIfNotFound
+  defaultVersionIfNotFound,
+  pinToExactVersion
 ) {
   for (const moduleName of fs.readdirSync(modulesDir)) {
     let version = versionsMap.get(moduleName);
@@ -199,14 +221,18 @@ function updatePackageVersions(
       if (packageInfo.dependencies) {
         for (const dep of Object.keys(packageInfo.dependencies)) {
           if (versionsMap.has(dep)) {
-            packageInfo.dependencies[dep] = version;
+            packageInfo.dependencies[dep] = pinToExactVersion
+              ? version
+              : '^' + version;
           }
         }
       }
       if (packageInfo.peerDependencies) {
         for (const dep of Object.keys(packageInfo.peerDependencies)) {
           if (versionsMap.has(dep)) {
-            packageInfo.peerDependencies[dep] = version;
+            packageInfo.peerDependencies[dep] = pinToExactVersion
+              ? version
+              : '^' + version;
           }
         }
       }