nix hash path, and preperatory refactors

- `nix store add` supports text hashing

  With functional test ensuring it matches `builtins.toFile`.

- Factored-out flags for both commands

- Move all common reusable flags to `libcmd`

  - They are not part of the *definition* of the CLI infra, just a usag
    of it.

  - The `libstore` flag couldn't go in `args.hh` in libutil anyways,
    would be awkward for it to live alone

- Shuffle around `Cmd*` hierarchy so flags for deprecated commands don't
  end up on the new ones

src/libcmd/misc-store-flags.cc

@@ -0,0 +1,121 @@
+#include "misc-store-flags.hh"
+
+namespace nix::flag
+{
+
+static void hashFormatCompleter(AddCompletions & completions, size_t index, std::string_view prefix)
+{
+    for (auto & format : hashFormats) {
+        if (hasPrefix(format, prefix)) {
+            completions.add(format);
+        }
+    }
+}
+
+Args::Flag hashFormatWithDefault(std::string && longName, HashFormat * hf)
+{
+    assert(*hf == nix::HashFormat::SRI);
+    return Args::Flag {
+            .longName = std::move(longName),
+            .description = "Hash format (`base16`, `nix32`, `base64`, `sri`). Default: `sri`.",
+            .labels = {"hash-format"},
+            .handler = {[hf](std::string s) {
+                *hf = parseHashFormat(s);
+            }},
+            .completer = hashFormatCompleter,
+    };
+}
+
+Args::Flag hashFormatOpt(std::string && longName, std::optional<HashFormat> * ohf)
+{
+    return Args::Flag {
+            .longName = std::move(longName),
+            .description = "Hash format (`base16`, `nix32`, `base64`, `sri`).",
+            .labels = {"hash-format"},
+            .handler = {[ohf](std::string s) {
+                *ohf = std::optional<HashFormat>{parseHashFormat(s)};
+            }},
+            .completer = hashFormatCompleter,
+    };
+}
+
+static void hashAlgoCompleter(AddCompletions & completions, size_t index, std::string_view prefix)
+{
+    for (auto & algo : hashAlgorithms)
+        if (hasPrefix(algo, prefix))
+            completions.add(algo);
+}
+
+Args::Flag hashAlgo(std::string && longName, HashAlgorithm * ha)
+{
+    return Args::Flag {
+            .longName = std::move(longName),
+            .description = "Hash algorithm (`md5`, `sha1`, `sha256`, or `sha512`).",
+            .labels = {"hash-algo"},
+            .handler = {[ha](std::string s) {
+                *ha = parseHashAlgo(s);
+            }},
+            .completer = hashAlgoCompleter,
+    };
+}
+
+Args::Flag hashAlgoOpt(std::string && longName, std::optional<HashAlgorithm> * oha)
+{
+    return Args::Flag {
+            .longName = std::move(longName),
+            .description = "Hash algorithm (`md5`, `sha1`, `sha256`, or `sha512`). Can be omitted for SRI hashes.",
+            .labels = {"hash-algo"},
+            .handler = {[oha](std::string s) {
+                *oha = std::optional<HashAlgorithm>{parseHashAlgo(s)};
+            }},
+            .completer = hashAlgoCompleter,
+    };
+}
+
+Args::Flag fileIngestionMethod(FileIngestionMethod * method)
+{
+    return Args::Flag {
+        .longName  = "mode",
+        // FIXME indentation carefully made for context, this is messed up.
+        .description = R"(
+    How to compute the hash of the input.
+    One of:
+
+    - `nar` (the default): Serialises the input as an archive (following the [_Nix Archive Format_](https://edolstra.github.io/pubs/phd-thesis.pdf#page=101)) and passes that to the hash function.
+
+    - `flat`: Assumes that the input is a single file and directly passes it to the hash function;
+        )",
+        .labels = {"file-ingestion-method"},
+        .handler = {[method](std::string s) {
+            *method = parseFileIngestionMethod(s);
+        }},
+    };
+}
+
+Args::Flag contentAddressMethod(ContentAddressMethod * method)
+{
+    return Args::Flag {
+        .longName  = "mode",
+        // FIXME indentation carefully made for context, this is messed up.
+        .description = R"(
+    How to compute the content-address of the store object.
+    One of:
+
+    - `nar` (the default): Serialises the input as an archive (following the [_Nix Archive Format_](https://edolstra.github.io/pubs/phd-thesis.pdf#page=101)) and passes that to the hash function.
+
+    - `flat`: Assumes that the input is a single file and directly passes it to the hash function;
+
+    - `text`: Like `flat`, but used for
+      [derivations](@docroot@/glossary.md#store-derivation) serialized in store object and 
+      [`builtins.toFile`](@docroot@/language/builtins.html#builtins-toFile).
+      For advanced use-cases only;
+      for regular usage prefer `nar` and `flat.
+        )",
+        .labels = {"content-address-method"},
+        .handler = {[method](std::string s) {
+            *method = ContentAddressMethod::parse(s);
+        }},
+    };
+}
+
+}