In preparation for LRU caching, don't make the reader store the
variables itself. Instead, the functions return the read state
directly.
The now defunct read() function is removed separately

The FunctionCommand was required to reset the
reader, which is not necessary anymore.

These provides are not necessary for dependecy resolving.
But upcoming caching will reuse the provides and therefore require
consistent use of it.

Add an LRU cache to the state_reader so state is only reread when an
actual changing command has been executed.
After command execution the cache of the relevant types is cleared.

Mirrors are updated ad-hoc. No dependency-management is required.
Therefore no Command is generated and the `mirror` dependency-type
doesn't exist.