buildNpmPackage: allow passing npmDeps

doc/languages-frameworks/javascript.section.md

···
 * `npmPruneFlags`: Flags to pass to `npm prune`. Defaults to the value of `npmInstallFlags`.
 * `makeWrapperArgs`: Flags to pass to `makeWrapper`, added to executable calling the generated `.js` with `node` as an interpreter. These scripts are defined in `package.json`.
 * `nodejs`: The `nodejs` package to build against, using the corresponding `npm` shipped with that version of `node`. Defaults to `pkgs.nodejs`.
+* `npmDeps`: The dependencies used to build the npm package. Especially useful to not have to recompute workspace depedencies.
 
 #### prefetch-npm-deps {#javascript-buildNpmPackage-prefetch-npm-deps}
 

pkgs/build-support/node/build-npm-package/default.nix

···
   # Value for npm `--workspace` flag and directory in which the files to be installed are found.
 , npmWorkspace ? null
 , nodejs ? topLevelArgs.nodejs
+, npmDeps ?  fetchNpmDeps {
+  inherit forceGitDeps src srcs sourceRoot prePatch patches postPatch;
+  name = "${name}-npm-deps";
+  hash = npmDepsHash;
+}
 , ...
 } @ args:
 
 let
-  npmDeps = fetchNpmDeps {
-    inherit forceGitDeps src srcs sourceRoot prePatch patches postPatch;
-    name = "${name}-npm-deps";
-    hash = npmDepsHash;
-  };
-
   # .override {} negates splicing, so we need to use buildPackages explicitly
   npmHooks = buildPackages.npmHooks.override {
     inherit nodejs;