[android][gradle] Use Node's module resolution algo to find JSC & Hermes

The Gradle build file looks up jsc-android and hermes-engine using hard-coded paths. Rather than assuming the location of these packages, which are distributed and installed as npm packages, this commit makes the Gradle file use Node's standard module resolution algorithm. It looks up the file hierarchy until it finds a matching npm package or reaches the root directory.

Test plan: Ensure that CI tests pass, and that `./gradlew :ReactAndroid:installArchives` works. Printed out the paths that the Gradle script found for jsc-android and hermes-engine (both were `<my stuff>/react-native/node_modules/jsc-android|hermes-engine`).

Author: ide

ReactAndroid/build.gradle

@@ -9,6 +9,8 @@ plugins {
     id("de.undercouch.download")
 }
 
+import java.nio.file.Paths
+
 import de.undercouch.gradle.tasks.download.Download
 import org.apache.tools.ant.taskdefs.condition.Os
 import org.apache.tools.ant.filters.ReplaceTokens
@@ -92,8 +94,14 @@ task prepareFolly(dependsOn: dependenciesPath ? [] : [downloadFolly], type: Copy
 }
 
 task prepareHermes() {
-    def hermesAAR = file("$projectDir/../node_modules/hermes-engine/android/hermes-debug.aar")
-    if (!hermesAAR.exists()) {
+    def hermesAAR = null
+    def hermesPackagePath = findNodeModulePath(projectDir, "hermes-engine")
+    if (hermesPackagePath) {
+        hermesAAR = file("$hermesPackagePath/android/hermes-debug.aar")
+        if (!hermesAAR.exists()) {
+            throw new GradleScriptException("The hermes-engine npm package is missing \"android/hermes-debug.aar\"")
+        }
+    } else {
         // For an app to build from RN source, hermes-engine is located at /path/to/app/node_modules
         // and $projectDir is located at /path/to/app/node_modules/react-native/ReactAndroid
         hermesAAR = file("$projectDir/../../hermes-engine/android/hermes-debug.aar")
@@ -160,16 +168,22 @@ task prepareGlog(dependsOn: dependenciesPath ? [] : [downloadGlog], type: Copy)
 // Create Android.mk library module based on jsc from npm
 task prepareJSC {
     doLast {
-        def jscPackageRoot = file("$projectDir/../node_modules/jsc-android/dist")
-        if (!jscPackageRoot.exists()) {
+        def jscDist = null
+        def jscPackagePath = findNodeModulePath(projectDir, "jsc-android")
+        if (jscPackagePath) {
+            jscDist = file("$jscPackagePath/dist")
+            if (!jscDist.exists()) {
+                throw new GradleScriptException("The jsc-android npm package is missing its \"dist\" directory")
+            }
+        } else {
             // For an app to build from RN source, the jsc-android is located at /path/to/app/node_modules
             // and $projectDir may located at /path/to/app/node_modules/react-native/ReactAndroid
-            jscPackageRoot = file("$projectDir/../../jsc-android/dist")
+            jscDist = file("$projectDir/../../jsc-android/dist")
         }
-        def jscAAR = fileTree(jscPackageRoot).matching({ it.include "**/android-jsc/**/*.aar" }).singleFile
+        def jscAAR = fileTree(jscDist).matching({ it.include "**/android-jsc/**/*.aar" }).singleFile
         def soFiles = zipTree(jscAAR).matching({ it.include "**/*.so" })
 
-        def headerFiles = fileTree(jscPackageRoot).matching({ it.include "**/include/*.h" })
+        def headerFiles = fileTree(jscDist).matching({ it.include "**/include/*.h" })
 
         copy {
             from(soFiles)
@@ -192,6 +206,20 @@ task downloadNdkBuildDependencies {
     dependsOn(downloadGlog)
 }
 
+def findNodeModulePath(baseDir, packageName) {
+    def basePath = baseDir.toPath().normalize()
+    // Node's module resolution algorithm searches up to the root directory,
+    // after which the base path will be null 
+    while (basePath) {
+        def candidatePath = Paths.get(basePath.toString(), "node_modules", packageName)
+        if (candidatePath.toFile().exists()) {
+            return candidatePath.toString()
+        }
+        basePath = basePath.getParent()
+    }
+    return null
+}
+
 def getNdkBuildName() {
     if (Os.isFamily(Os.FAMILY_WINDOWS)) {
         return "ndk-build.cmd"