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/executable-path.hh

@@ -0,0 +1,81 @@
+#pragma once
+///@file
+
+#include "nix/util/file-system.hh"
+
+namespace nix {
+
+MakeError(ExecutableLookupError, Error);
+
+/**
+ * @todo rename, it is not just good for execuatable paths, but also
+ * other lists of paths.
+ */
+struct ExecutablePath
+{
+    std::vector<std::filesystem::path> directories;
+
+    constexpr static const OsChar separator =
+#ifdef WIN32
+        L';'
+#else
+        ':'
+#endif
+        ;
+
+    /**
+     * Parse `path` into a list of paths.
+     *
+     * On Unix we split on `:`, on Windows we split on `;`.
+     *
+     * For Unix, this is according to the POSIX spec for `PATH`.
+     * https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
+     */
+    static ExecutablePath parse(const OsString & path);
+
+    /**
+     * Load the `PATH` environment variable and `parse` it.
+     */
+    static ExecutablePath load();
+
+    /**
+     * Opposite of `parse`
+     */
+    OsString render() const;
+
+    /**
+     * Search for an executable.
+     *
+     * For Unix, this is according to the POSIX spec for `PATH`.
+     * https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
+     *
+     * @param exe This must just be a name, and not contain any `/` (or
+     * `\` on Windows). in case it does, per the spec no lookup should
+     * be perfomed, and the path (it is not just a file name) as is.
+     * This is the caller's respsonsibility.
+     *
+     * This is a pure function, except for the default `isExecutable`
+     * argument, which uses the ambient file system to check if a file is
+     * executable (and exists).
+     *
+     * @return path to a resolved executable
+     */
+    std::optional<std::filesystem::path> findName(
+        const OsString & exe,
+        std::function<bool(const std::filesystem::path &)> isExecutableFile = isExecutableFileAmbient) const;
+
+    /**
+     * Like the `findName` but also allows a file path as input.
+     *
+     * This implements the full POSIX spec: if the path is just a name,
+     * it searches like the above. Otherwise, it returns the path as is.
+     * If (in the name case) the search fails, an exception is thrown.
+     */
+    std::filesystem::path findPath(
+        const std::filesystem::path & exe,
+        std::function<bool(const std::filesystem::path &)> isExecutable = isExecutableFileAmbient) const;
+
+    bool operator==(const ExecutablePath &) const = default;
+};
+
+} // namespace nix