jar files are exported for each Java action even when they are shared

When running `wskdeploy export`, the tool downloads all artefacts related to the project's package. If the project has multiple Java actions that are packaged in the same jar file, it downloads that jar file several times under different names.

## Steps to reproduce

1. Create 2 Java actions (named `action1` and `action2`) packaged in the same jar file (named `myactions.jar`)
2. Create a manifest to deploy those in a package called `example`
3. Run `wskdeploy sync`
4. Run `wskdeploy export`

## Actual behaviour

`wskdeploy` creates a folder `example` and in that folder it downloads the jar file twice as `action1.jar` and `action2.jar`.

## Expected behaviour

`wskdeploy` should work out that there is only one jar file and download it only once.

From a functional point of view, the current behaviour works, it impacts the export's performance in terms of time spent and local disk space used.

Start at the `wskdeploy export` flow and trace how package artefacts are downloaded and named for multiple Java actions. Reproduce the case with two actions sharing `myactions.jar`, then verify that the exported `example` folder contains one jar while both actions remain usable.

Written by the indexing model from the issue text.