Convert to multi-module project

[Nashatyrev]

The aim is to make this project multi-module to later add the gossip simulator module which now lives in tests in a separate branch.

• extract the main module as a libp2p submodule
• move specific dependencies and gradle plugins to the libp2p module
• remove unused dependencies
• add examples as submodules and adjust their code
• android example is still out of the scope as I couldn't make it working in a reasonable time. Need to file an issue on this.
• modules which want their artifacts to be published should define mavenArtifactId project property. Now just libp2p module is published under the old artifact id jvm-libp2p-minimal
• remove 'log4j-core' from 'libp2p' module: it's needed in runtime for tests or applications only
• extract dependency versions to the dedicated file

This PR was inspired by #198, thanks to @SalvatoreT

[Nashatyrev]

@StefanBratanov had approved this PR