feat: add yarnpkg/core sub exports

[ChALkeR]

What's the problem this PR addresses?

Same reason as #6611

Those are already fully exported here:

berry/packages/yarnpkg-core/sources/index.ts

Lines 9 to 10 in 8bfe2d5

	import * as semverUtils from './semverUtils';
	import * as structUtils from './structUtils';

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

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.