Cleanup Resource Manager a bit to increase clarity. (backport #816)

[mergify]

This is an automatic backport of pull request #816 done by Mergify.
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	both modified:   hardware_interface/include/hardware_interface/resource_manager.hpp
	both modified:   hardware_interface/src/resource_manager.cpp
	both modified:   hardware_interface/test/mock_components/test_generic_system.cpp
	both modified:   hardware_interface/test/test_resource_manager.cpp

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[christophfroehlich]

Needs #837 first.

[mergify]

This pull request is in conflict. Could you fix it @mergify[bot]?

[christophfroehlich]

@Mergifyio rebase

[mergify]

rebase

☑️ Nothing to do

• -conflict [:pushpin: rebase requirement]
• -closed [:pushpin: rebase requirement]

[christophfroehlich]

Diff is smaller than the original PR, because some changes were already backported, e.g., with #1051