Add buildSuccess file to avoid over-zealous cache updates

[hanslovsky]

Before, caching would only work if no main class was specified but inferred from the jar instead.

buildSuccess file specified in #31

Addresses #31 for Python jgo:

• directory structure/tree of cache
• mainClass file name when main class is specified in the endpoint
• buildSuccess file to indicate successful build

[hanslovsky]

With commit 596e32a, I now have:

$ rm -rf ~/.jgo
jgo org.codehaus.groovy:groovy-groovysh:org.codehaus.groovy.tools.shell.Main+commons-cli:commons-cli:1.3.1
<cache-root>
└── org.codehaus.groovy
    └── groovy-groovysh
        └── RELEASE+commons-cli-commons-cli-1.3.1
            ├── buildSuccess
            ├── commons-cli-1.3.1.jar
            ├── groovy-3.0.0-beta-3.jar
            ├── groovy-console-3.0.0-beta-3.jar
            ├── groovy-groovysh-3.0.0-beta-3.jar
            ├── groovy-swing-3.0.0-beta-3.jar
            ├── groovy-templates-3.0.0-beta-3.jar
            ├── groovy-xml-3.0.0-beta-3.jar
            ├── jline-2.14.6.jar
            ├── org.codehaus.groovy.tools.shell.Main
            └── pom.xml

[hanslovsky]

@ctrueden can I go ahead and merge this? I would like to publish a release with this fix ASAP because current the latest Python jgo release always updates the cache directory when a mainClass is specified (and not inferred from the mainClass in the manifest), i.e.

jgo gId:aId:mainClass

is always run as

jgo -u gId:aId:mainClass

I am confident about these changes and if you are busy at the moment I can merge without your review if you are ok with that.

[ctrueden]

@hanslovsky Please go ahead. I have lots going on right now.

Doing my jgo issues is still high on my to-do list, but I don't know if I'll get to it before the September hackathon. I will try.

[hanslovsky]

Thanks @ctrueden
For me personally, only this one is really high priority, because current behavior basically renders caching useless.