feat: add yarnpkg/core sub exports (#6614)

## What's the problem this PR addresses?

Same reason as #6611 

Those are already fully exported here:
https://github.com/yarnpkg/berry/blob/8bfe2d545e986993e4450072bac8b1044e5ebed7/packages/yarnpkg-core/sources/index.ts#L9-L10

But importing them from top pulls in all the yarpkg/core subdeps, while
those two are pretty light on their own and don't need all that

## How did you fix it?

Adding them as separate exports would fix this

## Checklist

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.yarn/versions/30ae755e.yml

@@ -0,0 +1,35 @@
+releases:
+  "@yarnpkg/core": patch
+
+declined:
+  - "@yarnpkg/plugin-compat"
+  - "@yarnpkg/plugin-constraints"
+  - "@yarnpkg/plugin-dlx"
+  - "@yarnpkg/plugin-essentials"
+  - "@yarnpkg/plugin-exec"
+  - "@yarnpkg/plugin-file"
+  - "@yarnpkg/plugin-git"
+  - "@yarnpkg/plugin-github"
+  - "@yarnpkg/plugin-http"
+  - "@yarnpkg/plugin-init"
+  - "@yarnpkg/plugin-interactive-tools"
+  - "@yarnpkg/plugin-link"
+  - "@yarnpkg/plugin-nm"
+  - "@yarnpkg/plugin-npm"
+  - "@yarnpkg/plugin-npm-cli"
+  - "@yarnpkg/plugin-pack"
+  - "@yarnpkg/plugin-patch"
+  - "@yarnpkg/plugin-pnp"
+  - "@yarnpkg/plugin-pnpm"
+  - "@yarnpkg/plugin-stage"
+  - "@yarnpkg/plugin-typescript"
+  - "@yarnpkg/plugin-version"
+  - "@yarnpkg/plugin-workspace-tools"
+  - "@yarnpkg/builder"
+  - "@yarnpkg/cli"
+  - "@yarnpkg/doctor"
+  - "@yarnpkg/extensions"
+  - "@yarnpkg/nm"
+  - "@yarnpkg/pnpify"
+  - "@yarnpkg/sdks"
+  - "@yarnpkg/shell"

packages/yarnpkg-core/package.json

@@ -5,6 +5,8 @@
   "main": "./sources/index.ts",
   "exports": {
     ".": "./sources/index.ts",
+    "./structUtils": "./sources/structUtils.ts",
+    "./semverUtils": "./sources/semverUtils.ts",
     "./package.json": "./package.json"
   },
   "sideEffects": false,
@@ -69,6 +71,8 @@
     "main": "./lib/index.js",
     "exports": {
       ".": "./lib/index.js",
+      "./structUtils": "./lib/structUtils.js",
+      "./semverUtils": "./lib/semverUtils.js",
       "./package.json": "./package.json"
     }
   },