Split out store-open.hh and store-registration.hh

The existing header is a bit too big. Now the following use-cases are
separated, and get their own headers:

- Using or implementing an arbitrary store: remaining `store-api.hh`

  This is closer to just being about the `Store` (and `StoreConfig`)
  classes, as one would expect.

- Opening a store from a textual description: `store-open.hh`

  Opening an aribtrary store implementation like this requires some sort
  of store registration mechanism to exists, but the caller doesn't need
  to know how it works. This just exposes the functions which use such a
  mechanism, without exposing the mechanism itself

- Registering a store implementation: `store-registration.hh`

  This requires understanding how the mechanism actually works, and the
  mechanism in question involves templated machinery in headers we
  rather not expose to things that don't need it, as it would slow down
  compilation for no reason.

src/libstore/store-api.cc

@@ -5,6 +5,7 @@
 #include "nix/store/realisation.hh"
 #include "nix/store/derivations.hh"
 #include "nix/store/store-api.hh"
+#include "nix/store/store-open.hh"
 #include "nix/util/util.hh"
 #include "nix/store/nar-info-disk-cache.hh"
 #include "nix/util/thread-pool.hh"
@@ -1304,108 +1305,3 @@ void Store::signRealisation(Realisation & realisation)
 }
 
 }
-
-
-#include "nix/store/local-store.hh"
-#include "nix/store/uds-remote-store.hh"
-
-
-namespace nix {
-
-ref<Store> openStore(const std::string & uri,
-    const Store::Config::Params & extraParams)
-{
-    return openStore(StoreReference::parse(uri, extraParams));
-}
-
-ref<Store> openStore(StoreReference && storeURI)
-{
-    auto & params = storeURI.params;
-
-    auto storeConfig = std::visit(overloaded {
-        [&](const StoreReference::Auto &) -> ref<StoreConfig> {
-            auto stateDir = getOr(params, "state", settings.nixStateDir);
-            if (access(stateDir.c_str(), R_OK | W_OK) == 0)
-                return make_ref<LocalStore::Config>(params);
-            else if (pathExists(settings.nixDaemonSocketFile))
-                return make_ref<UDSRemoteStore::Config>(params);
-            #ifdef __linux__
-            else if (!pathExists(stateDir)
-                && params.empty()
-                && !isRootUser()
-                && !getEnv("NIX_STORE_DIR").has_value()
-                && !getEnv("NIX_STATE_DIR").has_value())
-            {
-                /* If /nix doesn't exist, there is no daemon socket, and
-                   we're not root, then automatically set up a chroot
-                   store in ~/.local/share/nix/root. */
-                auto chrootStore = getDataDir() + "/root";
-                if (!pathExists(chrootStore)) {
-                    try {
-                        createDirs(chrootStore);
-                    } catch (SystemError & e) {
-                        return make_ref<LocalStore::Config>(params);
-                    }
-                    warn("'%s' does not exist, so Nix will use '%s' as a chroot store", stateDir, chrootStore);
-                } else
-                    debug("'%s' does not exist, so Nix will use '%s' as a chroot store", stateDir, chrootStore);
-                return make_ref<LocalStore::Config>("local", chrootStore, params);
-            }
-            #endif
-            else
-                return make_ref<LocalStore::Config>(params);
-        },
-        [&](const StoreReference::Specified & g) {
-            for (const auto & [storeName, implem] : Implementations::registered())
-                if (implem.uriSchemes.count(g.scheme))
-                    return implem.parseConfig(g.scheme, g.authority, params);
-
-            throw Error("don't know how to open Nix store with scheme '%s'", g.scheme);
-        },
-    }, storeURI.variant);
-
-    experimentalFeatureSettings.require(storeConfig->experimentalFeature());
-    storeConfig->warnUnknownSettings();
-
-    auto store = storeConfig->openStore();
-    store->init();
-
-    return store;
-}
-
-std::list<ref<Store>> getDefaultSubstituters()
-{
-    static auto stores([]() {
-        std::list<ref<Store>> stores;
-
-        StringSet done;
-
-        auto addStore = [&](const std::string & uri) {
-            if (!done.insert(uri).second) return;
-            try {
-                stores.push_back(openStore(uri));
-            } catch (Error & e) {
-                logWarning(e.info());
-            }
-        };
-
-        for (const auto & uri : settings.substituters.get())
-            addStore(uri);
-
-        stores.sort([](ref<Store> & a, ref<Store> & b) {
-            return a->config.priority < b->config.priority;
-        });
-
-        return stores;
-    } ());
-
-    return stores;
-}
-
-Implementations::Map & Implementations::registered()
-{
-    static Map registered;
-    return registered;
-}
-
-}