JavaMavenWorkflow sam build fails with `[Errno 20] Not a directory`

Getting an error when running `sam build` in an existing project of mine,
where the sam template has a CodeUri that points to a JAR file in a reactor build submodule.

**Steps to reproduce the issue:**

1. existing java maven project using reactor builds with
1.1 a shaded jar generated in a maven submodule's target directory
(`/lambdas/mainlambda/target/lambda.jar` in my case)
1.2 a template.yaml in the `` directory using
`CodeUri: lambdas/mainlambda/target/lambda.jar`
2. cd ``
3. `sam build --debug`

**Observed result:**
```text
2019-04-28 19:31:55 Collected default values for parameters: {}
2019-04-28 19:31:55 12 resources found in the template
2019-04-28 19:31:55 Found Serverless function with name='LatestFunction' and CodeUri='lambdas/mainlambda/target/lambda.jar'
2019-04-28 19:31:55 Found Serverless function with name='LiveFunction' and CodeUri='lambdas/mainlambda/target/lambda.jar'
2019-04-28 19:31:55 Found Serverless function with name='TestRunnerFunction' and CodeUri='lambdas/testlambda/target/lambda.jar'
2019-04-28 19:31:55 Building resource 'LatestFunction'
2019-04-28 19:31:55 Looking for a supported build workflow in following directories: ['/GitRepos/someproject/lambdas/mainlambda/target/lambda.jar', '/GitRepos/someproject']
2019-04-28 19:31:55 Loading workflow module 'aws_lambda_builders.workflows'
2019-04-28 19:31:55 Registering workflow 'PythonPipBuilder' with capability 'Capability(language='python', dependency_manager='pip', application_framework=None)'
2019-04-28 19:31:55 Registering workflow 'NodejsNpmBuilder' with capability 'Capability(language='nodejs', dependency_manager='npm', application_framework=None)'
2019-04-28 19:31:55 Registering workflow 'RubyBundlerBuilder' with capability 'Capability(language='ruby', dependency_manager='bundler', application_framework=None)'
2019-04-28 19:31:55 Registering workflow 'GoDepBuilder' with capability 'Capability(language='go', dependency_manager='dep', application_framework=None)'
2019-04-28 19:31:55 Registering workflow 'GoModulesBuilder' with capability 'Capability(language='go', dependency_manager='modules', application_framework=None)'
2019-04-28 19:31:55 Registering workflow 'JavaGradleWorkflow' with capability 'Capability(language='java', dependency_manager='gradle', application_framework=None)'
2019-04-28 19:31:55 Registering workflow 'JavaMavenWorkflow' with capability 'Capability(language='java', dependency_manager='maven', application_framework=None)'
2019-04-28 19:31:55 Registering workflow 'DotnetCliPackageBuilder' with capability 'Capability(language='dotnet', dependency_manager='cli-package', application_framework=None)'
2019-04-28 19:31:55 Found workflow 'JavaMavenWorkflow' to support capabilities 'Capability(language='java', dependency_manager='maven', application_framework=None)'
2019-04-28 19:31:55 Running workflow 'JavaMavenWorkflow'
2019-04-28 19:31:55 Running JavaMavenWorkflow:CopySource
2019-04-28 19:31:55 JavaMavenWorkflow:CopySource raised unhandled exception
Traceback (most recent call last):
File "/.virtualenvs/aws/lib/python3.6/site-packages/aws_lambda_builders/workflow.py", line 248, in run
action.execute()
File "/.virtualenvs/aws/lib/python3.6/site-packages/aws_lambda_builders/actions.py", line 101, in execute
copytree(self.source_dir, self.dest_dir, ignore=shutil.ignore_patterns(*self.excludes))
File "/.virtualenvs/aws/lib/python3.6/site-packages/aws_lambda_builders/utils.py", line 43, in copytree
names = os.listdir(source)
NotADirectoryError: [Errno 20] Not a directory: '/GitRepos/someproject/lambdas/mainlambda/target/lambda.jar'
```

**Expected result:**

I would expect it to run the equivalent of

```bash
(in folder)
mvn package -Dmaven.test.skip=true -pl lambdas/mainlambda -am
```
Which is what i run when I want to just build lambda.jar

- `-pl lambdas/mainlambda` because that is the module folder of the `target` folder where `lambda.jar` is located.
- `-am` because otherwise reactor builds will fail
- `-Dmaven.test.skip=true` because when run from the aws-toolkit, you don't want to rerun the whole test suite (?)

This `sam build` failure is preventing me from creating launch configurations using aws-toolkit for Intellij.

**Additional environment details (Ex: Windows, Mac, Amazon Linux etc)**

I don't believe my environment makes any difference in this issue, but I'll answer any request for more information if that is deemed necessary