Expose the nix component in header include paths

For example, instead of doing

    #include "nix/store-config.hh"
    #include "nix/derived-path.hh"

Now do

    #include "nix/store/config.hh"
    #include "nix/store/derived-path.hh"

This was originally planned in the issue, and also recent requested by
Eelco.

Most of the change is purely mechanical. There is just one small
additional issue. See how, in the example above, we took this
opportunity to also turn `<comp>-config.hh` into `<comp>/config.hh`.
Well, there was already a `nix/util/config.{cc,hh}`. Even though there
is not a public configuration header for libutil (which also would be
called `nix/util/config.{cc,hh}`) that's still confusing, To avoid any
such confusion, we renamed that to `nix/util/configuration.{cc,hh}`.

Finally, note that the libflake headers already did this, so we didn't
need to do anything to them. We wouldn't want to mistakenly get
`nix/flake/flake/flake.hh`!

Progress on #7876

src/libutil/include/nix/util/file-content-address.hh

@@ -0,0 +1,160 @@
+#pragma once
+///@file
+
+#include "nix/util/source-accessor.hh"
+
+namespace nix {
+
+struct SourcePath;
+
+/**
+ * An enumeration of the ways we can serialize file system
+ * objects.
+ *
+ * See `file-system-object/content-address.md#serial` in the manual for
+ * a user-facing description of this concept, but note that this type is also
+ * used for storing or sending copies; not just for addressing.
+ * Note also that there are other content addressing methods that don't
+ * correspond to a serialisation method.
+ */
+enum struct FileSerialisationMethod : uint8_t {
+    /**
+     * Flat-file. The contents of a single file exactly.
+     *
+     * See `file-system-object/content-address.md#serial-flat` in the
+     * manual.
+     */
+    Flat,
+
+    /**
+     * Nix Archive. Serializes the file-system object in
+     * Nix Archive format.
+     *
+     * See `file-system-object/content-address.md#serial-nix-archive` in
+     * the manual.
+     */
+    NixArchive,
+};
+
+/**
+ * Parse a `FileSerialisationMethod` by name. Choice of:
+ *
+ *  - `flat`: `FileSerialisationMethod::Flat`
+ *  - `nar`: `FileSerialisationMethod::NixArchive`
+ *
+ * Opposite of `renderFileSerialisationMethod`.
+ */
+FileSerialisationMethod parseFileSerialisationMethod(std::string_view input);
+
+/**
+ * Render a `FileSerialisationMethod` by name.
+ *
+ * Opposite of `parseFileSerialisationMethod`.
+ */
+std::string_view renderFileSerialisationMethod(FileSerialisationMethod method);
+
+/**
+ * Dump a serialization of the given file system object.
+ */
+void dumpPath(
+    const SourcePath & path,
+    Sink & sink,
+    FileSerialisationMethod method,
+    PathFilter & filter = defaultPathFilter);
+
+/**
+ * Restore a serialisation of the given file system object.
+ *
+ * \todo use an arbitrary `FileSystemObjectSink`.
+ */
+void restorePath(
+    const Path & path,
+    Source & source,
+    FileSerialisationMethod method,
+    bool startFsync = false);
+
+
+/**
+ * Compute the hash of the given file system object according to the
+ * given method.
+ *
+ * the hash is defined as (in pseudocode):
+ *
+ * ```
+ * hashString(ha, dumpPath(...))
+ * ```
+ */
+HashResult hashPath(
+    const SourcePath & path,
+    FileSerialisationMethod method, HashAlgorithm ha,
+    PathFilter & filter = defaultPathFilter);
+
+/**
+ * An enumeration of the ways we can ingest file system
+ * objects, producing a hash or digest.
+ *
+ * See `file-system-object/content-address.md` in the manual for a
+ * user-facing description of this concept.
+ */
+enum struct FileIngestionMethod : uint8_t {
+    /**
+     * Hash `FileSerialisationMethod::Flat` serialisation.
+     *
+     * See `file-system-object/content-address.md#serial-flat` in the
+     * manual.
+     */
+    Flat,
+
+    /**
+     * Hash `FileSerialisationMethod::NixArchive` serialisation.
+     *
+     * See `file-system-object/content-address.md#serial-flat` in the
+     * manual.
+     */
+    NixArchive,
+
+    /**
+     * Git hashing.
+     *
+     * Part of `ExperimentalFeature::GitHashing`.
+     *
+     * See `file-system-object/content-address.md#serial-git` in the
+     * manual.
+     */
+    Git,
+};
+
+/**
+ * Parse a `FileIngestionMethod` by name. Choice of:
+ *
+ *  - `flat`: `FileIngestionMethod::Flat`
+ *  - `nar`: `FileIngestionMethod::NixArchive`
+ *  - `git`: `FileIngestionMethod::Git`
+ *
+ * Opposite of `renderFileIngestionMethod`.
+ */
+FileIngestionMethod parseFileIngestionMethod(std::string_view input);
+
+/**
+ * Render a `FileIngestionMethod` by name.
+ *
+ * Opposite of `parseFileIngestionMethod`.
+ */
+std::string_view renderFileIngestionMethod(FileIngestionMethod method);
+
+/**
+ * Compute the hash of the given file system object according to the
+ * given method, and for some ingestion methods, the size of the
+ * serialisation.
+ *
+ * Unlike the other `hashPath`, this works on an arbitrary
+ * `FileIngestionMethod` instead of `FileSerialisationMethod`, but
+ * may not return the size as this is this is not a both simple and
+ * useful defined for a merkle format.
+ */
+std::pair<Hash, std::optional<uint64_t>> hashPath(
+    const SourcePath & path,
+    FileIngestionMethod method, HashAlgorithm ha,
+    PathFilter & filter = defaultPathFilter);
+
+}