Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/executable-path.hh

@@ -0,0 +1,81 @@
+#pragma once
+///@file
+
+#include "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