Move everything except public functionality to detail namespace

[msimberg]

The public API of pika is small: sender/receiver functionality, runtime initialization, what else?

Hidden functionality can then gradually be brought into the public namespace through pika::experimental:: or directly into pika::.

The only reasonable way to do this is module by module:

• affinity (Clean up affinity module #152)
• algorithms (Clean up algorithms module part 1 #377, Move more algorithms/execution traits into detail namespace #411, More algorithms module namespace cleanup #475)
• allocator_support (Clean up allocator_support module #153)
• assertion (Clean up assertion module #155)
• async_base (Clean up async_base module #158)
• async_combinators (Clean up async_combinators module #160)
• async_cuda (Clean up async_cuda module #196)
• async (nothing to be done)
• async_mpi (Clean up async_mpi module #374)
• command_line_handling (Clean up command_line_handling module #216)
• concepts (nothing to be done)
• concurrency (Clean up concurrency module #246)
• config (Clean up config module #248)
• coroutines (Clean up coroutines module #257)
• datastructures (Clean up datastructures module #276)
• debugging (Clean up debugging module #324)
• errors (Clean up errors module #365)
• execution(Clean up namespaces etc. in execution modules #508)
• execution_base (Clean up namespaces etc. in execution modules #508)
• executors (Clean up namespaces etc. in execution modules #508)
• filesystem (Clean up filesystem module #379)
• format (Use fmt #487)
• functional (Clean up functional module #380)
• futures (Remove unused headers from futures module #525)
• hardware (Clean up hardware module #607)
• hashing (Clean up the hashing module #631)
• include (Clean up include module #632)
• ini (Move ini module functionality into detail namespace #633)
• init_runtime (Clean up init_runtime module #634)
• iterator_support
• itt_notify
• lock_registration
• logging
• memory
• mpi_base
• pack_traversal
• prefix (Cleanup prefix module - part 1 #1177)
• preprocessor
• program_options
• properties (Clean up properties module #673)
• resource_partitioner
• runtime_configuration
• runtime (Cleanup runtime module - part 1 #826, Cleanup runtime module - part 2 #1091)
• schedulers (Clean up schedulers module #625)
• string_util (Clean up string_util module #595)
• synchronization (Clean up synchronization #483)
• tag_invoke (Clean up tag_invoke module #596) (Replace pika's invoke_result with std::invoke_result #599)
• testing (Clean up testing module #594)
• thread_pool_util (Clean up thread_pool_util module #509)
• thread_pools (Clean up thread_pools module #462)
• thread_support (Clean up thread_support module #461)
• threading (nothing to be done)
• threading_base (Clean up threading base #445)
• threadmanager (Clean up threadmanager #428)
• timing (Clean up the timing module #209)
• topology (Clean up topology module #179)
• type_support (Clean up type support #386, Clean up type support part 2 #400)
• util (Clean up util #420)
• version (Clean up version module #166)

This is also a good opportunity to do general cleanup.
Avoid nesting detail namespaces into experimental namespace #448