observable build allows files outside of the source root to be copied

[mbostock]

This appears to be a regression in #89 which removed the isLocalFile check here:

e715830#diff-af4191cdea0d822d0199df1884ab420fd59f9db884a921be717ee21b20028248

As an example, consider this docs/index.md:

```js
const data = FileAttachment("../README.md").text();
```

Running observable build results in:

% yarn build
yarn run v1.22.19
$ rm -rf dist && ./bin/observable.ts build
render docs/contributing.md → dist/contributing.html
render docs/index.md → dist/index.html
render docs/javascript.md → dist/javascript.html
render docs/markdown.md → dist/markdown.html
copy public/client.js → dist/_observablehq/client.js
copy public/database.js → dist/_observablehq/database.js
copy public/style.css → dist/_observablehq/style.css
copy README.md → dist/README.md
copy docs/gistemp.csv → dist/_file/gistemp.csv
copy node_modules/@observablehq/runtime/dist/runtime.js → dist/_observablehq/runtime.js
✨  Done in 0.79s.

Note the copy README.md → dist/README.md. This should not be allowed.