fix: explain peer-default dependencies in why

[StantonMatt]

Closes #7159.

yarn why currently skips any dependency whose ident is also listed in peerDependencies, which hides real peer-with-default dependency edges. In the issue reproduction, @siteimprove/alfa-cypress declares cypress in both dependencies and peerDependencies, so yarn why cypress reports nothing unless --peers is set.

This keeps the default behavior of hiding injected virtual peer provisions, but no longer hides regular dependency edges just because the package also has a peer declaration for the same ident.

Verification:

• TEST_BINARY=scripts/run-yarn.js node scripts/run-yarn.js workspace acceptance-tests test:integration packages/acceptance-tests/pkg-tests-specs/sources/commands/why.test.ts --runInBand
• YARN_IGNORE_PATH=1 node scripts/run-yarn.js why cypress in a clone of Siteimprove/alfa-integrations now prints @siteimprove/alfa-cypress -> cypress
• YARN_IGNORE_PATH=1 node scripts/run-yarn.js why -R yauzl in the same clone now includes the Cypress branch
• node scripts/run-yarn.js test:lint
• node scripts/run-yarn.js typecheck:all
• node scripts/run-yarn.js version check
• git diff --check