use relative imports in miniflare core

[ajwootto]

• switch @miniflare/core to use relative imports to its own files rather than self-referencing its package name
• should fix usage in pnpm-style package installation environments

fixes #663

[changeset-bot]

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Latest commit: 00c17ea

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[mrbbot]

Hey! 👋 I've just released version 2.14.1 which includes this change. Thanks again for the PR!