fix: match import glob common base by path segment

[jiyujie2006]

Description

getCommonBase currently checks common directories with startsWith, so /a/foo is treated as a common base for /a/foo and /a/foobar. This can pass a too-narrow cwd to glob matching when directory names share a string prefix.

This changes the check to require a full path segment match, and adds coverage for the prefix case.

Additional context

Verified with:

• pnpm --filter vite build
• pnpm exec vitest run packages/vite/src/node/__tests__/plugins/importGlob/utils.spec.ts
• pnpm exec eslint --no-cache packages/vite/src/node/plugins/importMetaGlob.ts packages/vite/src/node/__tests__/plugins/importGlob/utils.spec.ts

[sapphi-red]

Would you also add an e2e test to https://github.com/vitejs/vite/blob/main/playground/glob-import/__tests__/glob-import.spec.ts ?