Combine AbstractPos, PosAdapter, and Pos

Also move `SourcePath` into `libutil`.

These changes allow `error.hh` and `error.cc` to access source path and
position information, which we can use to produce better error messages
(for example, we could consider omitting filenames when two or more
consecutive stack frames originate from the same file).

src/libfetchers/fetch-to-store.cc

@@ -0,0 +1,68 @@
+#include "fetch-to-store.hh"
+#include "fetchers.hh"
+#include "cache.hh"
+
+namespace nix {
+
+StorePath fetchToStore(
+    Store & store,
+    const SourcePath & path,
+    std::string_view name,
+    ContentAddressMethod method,
+    PathFilter * filter,
+    RepairFlag repair)
+{
+    // FIXME: add an optimisation for the case where the accessor is
+    // an FSInputAccessor pointing to a store path.
+
+    std::optional<fetchers::Attrs> cacheKey;
+
+    if (!filter && path.accessor->fingerprint) {
+        cacheKey = fetchers::Attrs{
+            {"_what", "fetchToStore"},
+            {"store", store.storeDir},
+            {"name", std::string(name)},
+            {"fingerprint", *path.accessor->fingerprint},
+            {
+                "method",
+                std::visit(overloaded {
+                    [](const TextIngestionMethod &) {
+                        return "text";
+                    },
+                    [](const FileIngestionMethod & fim) {
+                        switch (fim) {
+                        case FileIngestionMethod::Flat: return "flat";
+                        case FileIngestionMethod::Recursive: return "nar";
+                        default: assert(false);
+                        }
+                    },
+                }, method.raw),
+            },
+            {"path", path.path.abs()}
+        };
+        if (auto res = fetchers::getCache()->lookup(store, *cacheKey)) {
+            debug("store path cache hit for '%s'", path);
+            return res->second;
+        }
+    } else
+        debug("source path '%s' is uncacheable", path);
+
+    Activity act(*logger, lvlChatty, actUnknown, fmt("copying '%s' to the store", path));
+
+    auto filter2 = filter ? *filter : defaultPathFilter;
+
+    auto storePath =
+        settings.readOnlyMode
+        ? store.computeStorePath(
+            name, *path.accessor, path.path, method, HashAlgorithm::SHA256, {}, filter2).first
+        : store.addToStore(
+            name, *path.accessor, path.path, method, HashAlgorithm::SHA256, {}, filter2, repair);
+
+    if (cacheKey)
+        fetchers::getCache()->add(store, *cacheKey, {}, storePath, true);
+
+    return storePath;
+}
+
+
+}