Buildpack should look for gradlew in the same place as build.gradle

[dmikusa]

Describe the Enhancement

Possible Solution

Presently, the buildpack looks for gradlew in the application root (i.e. /workspace/gradlew). If it doesn't exist, it falls back to installing the latest version of Gradle.

A case was reported where gradlew was not in the application root, but a sub-directory. It was, however, located next to build.gradle. After we check for gradlew in the app root, we should then check the directory where build.gradle exists (if it's different). Then fall back to installing gradle as we do now.

We might also want to consider checking the adjusted working directory (see #339) for a possible location of gradlew also.

This should work although there is a slight risk to backwards compatibility. If a user happens to have a build.gradle and a gradlew that was not previously being found, they'll experience different behavior. This seems like a very unlikely situation though and very low risk.

Motivation

Make more use cases "just work".