[FR] Support multiple source code directories

[gian2dchris]

Support multiple source code directories

Hello good people !

Is your feature request related to a problem? Please describe.
I am trying to run mariana-trench on an application that contains its source code in multiple directories and an external package. I ran into this problem while trying to integrate mariana trench into a build pipeline for mobile applications, but I think this feature also makes sense for scanning non-Android projects.

Repository structure example:

./app
├── config
├── docs
├── gradle
├── app-common
│   └── src
├── app-external
│   └── src
├── externalSDK.java
├── app-login
│   └── src
└── app-test
    └── src

Describe the solution you'd like
I would like to execute something similar to:

(mariana-trench)$ mariana-trench \  
--system-jar-configuration-path=$ANDROID_SDK/platforms/android-30/android.jar  
--apk-path=app-debug.apk \  
--source-root-directory=./app/app-common/src/main/java
--source-root-directory=./app/app-external/src/main/java
--source-root-directory=./app/app-login/src/main/java
--source-root-directory=./app/externalSDK.java/src/main/java

Additional context
I discussed this issue with some people from Facebook's bounty team, who presented Mariana Trench during BountyConEdu and it was suggested, that I open an issue here so that it gets handled by the software team responsible.

Any clarification required, let me know.

Regards,
Giannis