Rename FlakeCache to InputCache and move it to libfetchers

[edolstra]

Motivation

InputCache is now keyed on Inputs rather than FlakeRefs. This prepares it to be used by fetchTree (in a future PR).

Context

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[Ericson2314]

I am very happy to see functionality move from libflakes to libfetchers like this, but I don't want there to be a new global variable. Because there is no global fetchers::Settings in libfetchers, I don't think it is even correct to have such a global variable.

If the purpose of this is fetchTree then I think it is better to have one of these two alternatives:

1. EvalState owns a fetchers::InputCache. Separate EvalStates maintain their own cache.
2. EvalState has a references to a fetchers::InputCache, and libcmd has a global variable, just like it has global variables for fetch::Settings and EvalSettings (see src/libcmd/common-eval-args.cc). getEvalState hooks up everything properly.

[edolstra]

@Ericson2314 Okay, the InputCache is now owned by EvalState.

[Ericson2314]

Thank you!!

Believe this has been addressed?