Rename nimbus source directory

[pedromiguelmiranda]

Now that nimbus binary is renamed to nimbus_execution_client (#2728 ), directory name could also be renamed to nimbus_execution_client (or similar suggestion) and free both the binary and the source directory.

However, there is a trade off regarding log history. All files history under nimbus directory won't be easily accessible via git log command, only via git log --follow (some git clients already implement this, not default with git command tool).

This tradeoff shoud be investigated before the change.

[arnetheduck]

the case could be made for execution_chain or simply execution here, to complement beacon_chain that we use in eth2

[arnetheduck]

Also, the makefile target name should be updated to make nimbus_execution_client