Proposal to manage visibility of subcomponents

[NicolasFussberger]

The folder structure already highlights the intended visibility.
For example:

score/launch_manager/src/daemon/src/alive_monitor
├── BUILD
├── details                           // everything in here shall be private to the alive_monitor component
├── IAliveMonitor.hpp                 // API used by other components
├── ....                              // API used by other components
└── mock_supervision_factory.hpp      // API used by other components

To make this more explicit and to enforce it via bazel, this PR proposes to define two package groups in the top-level build file of the component:

# Packages allows to use the alive_monitor component API.
package_group(
    name = "api",
    packages = ["//score/launch_manager/src/daemon/..."],
)

# Everything else in this package is internal to the alive_monitor component.
# Visibility for everything in 'details' shall be :internal by default.
package_group(
    name = "internal",
    packages = ["//score/launch_manager/src/daemon/src/alive_monitor/..."],
)

The targets in score/launch_manager/src/daemon/src/alive_monitor/BUILD will get by default the visibility from "api" while the targets in score/launch_manager/src/daemon/src/alive_monitor/details will by default get visibility "internal".

The same pattern could be repeated for all (sub)components