Make the JSON format for derivation use basename store paths

See #13570 for details --- the idea is that included the store dir in
store paths makes systematic JSON parting with e.g. Serde, Aeson,
nlohmann, or similiar harder.

After talking to Eelco, we are changing the `Derivation` format right
away because not only is `nix derivation` technically experimental, we think it is
also less widely used in practice than, say, `nix path-info`.

Progress on #13570

doc/manual/rl-next/derivation-json.md

@@ -0,0 +1,17 @@
+---
+synopsis: Derivation JSON format now uses store path basenames (no store dir) only
+prs: [13980]
+issues: [13570]
+---
+
+Experience with many JSON frameworks (e.g. nlohmann/json in C++, Serde
+in Rust, and Aeson in Haskell), has show that the use of the store dir
+in JSON formats is an impediment to systematic JSON formats, because it
+requires the serializer/deserializer to take an extra paramater (the
+store dir).
+
+We ultimately want to rectify this issue with all (non-stable, able to
+be changed) JSON formats. To start with, we are changing the JSON format
+for derivations because the `nix derivation` commands are --- in
+addition to being formally unstable --- less widely used than other
+unstable commands.

doc/manual/source/protocols/json/derivation.md

@@ -14,6 +14,21 @@ is a JSON object with the following fields:
   The name of the derivation.
   This is used when calculating the store paths of the derivation's outputs.
 
+* `version`:
+  Must be `3`.
+  This is a guard that allows us to continue evolving this format.
+  The choice of `3` is fairly arbitrary, but corresponds to this informal version:
+
+  - Version 0: A-Term format
+
+  - Version 1: Original JSON format, with ugly `"r:sha256"` inherited from A-Term format.
+
+  - Version 2: Separate `method` and `hashAlgo` fields in output specs
+
+  - Verison 3: Drop store dir from store paths, just include base name.
+
+  Note that while this format is experimental, the maintenance of versions is best-effort, and not promised to identify every change.
+
 * `outputs`:
   Information about the output paths of the derivation.
   This is a JSON object with one member per output, where the key is the output name and the value is a JSON object with these fields:
@@ -52,7 +67,6 @@ is a JSON object with the following fields:
   > ```json
   > "outputs": {
   >   "out": {
-  >     "path": "/nix/store/2543j7c6jn75blc3drf4g5vhb1rhdq29-source",
   >     "method": "nar",
   >     "hashAlgo": "sha256",
   >     "hash": "6fc80dcc62179dbc12fc0b5881275898f93444833d21b89dfe5f7fbcbb1d0d62"
@@ -63,6 +77,15 @@ is a JSON object with the following fields:
 * `inputSrcs`:
   A list of store paths on which this derivation depends.
 
+  > **Example**
+  >
+  > ```json
+  > "inputSrcs": [
+  >   "47y241wqdhac3jm5l7nv0x4975mb1975-separate-debug-info.sh",
+  >   "56d0w71pjj9bdr363ym3wj1zkwyqq97j-fix-pop-var-context-error.patch"
+  > ]
+  > ```
+
 * `inputDrvs`:
   A JSON object specifying the derivations on which this derivation depends, and what outputs of those derivations.
 
@@ -70,8 +93,8 @@ is a JSON object with the following fields:
   >
   > ```json
   > "inputDrvs": {
-  >   "/nix/store/6lkh5yi7nlb7l6dr8fljlli5zfd9hq58-curl-7.73.0.drv": ["dev"],
-  >   "/nix/store/fn3kgnfzl5dzym26j8g907gq3kbm8bfh-unzip-6.0.drv": ["out"]
+  >   "6lkh5yi7nlb7l6dr8fljlli5zfd9hq58-curl-7.73.0.drv": ["dev"],
+  >   "fn3kgnfzl5dzym26j8g907gq3kbm8bfh-unzip-6.0.drv": ["out"]
   > }
   > ```