Incorrect CodeUri for sam build

https://github.com/aws-cloudformation/cloudformation-cli-java-plugin/blob/10e32d7b60f2395650900022e7d5e161481f5136/python/rpdk/java/codegen.py#L49

This `CodeUri` may be correct for other SAM commands, but if you run `sam build` it's incorrect. See https://github.com/awslabs/aws-sam-cli/issues/1278#issuecomment-512341533:

> It looks like your CodeUri is a jar? It should be the directory of your source not the compiled artifact.

The error it gives:

```
Build Failed - Error: JavaMavenWorkflow:CopySource - [Errno 20] Not a directory: '/usr/aws-logs-loggroup/target/aws-logs-loggroup-handler-1.0-SNAPSHOT.jar
```

I fixed it by manually changing the template file:

`CodeUri: ./target/aws-logs-loggroup-handler-1.0-SNAPSHOT.jar`

to

`CodeUri: .`

Start at python/rpdk/java/codegen.py#L49 and inspect how the generated template sets CodeUri. Run sam build against a generated Java project to reproduce the Not a directory error; done means the template uses the source directory and sam build completes successfully.

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