Linked sourcemap annotations are incorrect when publicPath is not absolute

When publicPath is relative, it gets prepended to the sourcemap annotation, and I think that's incorrect.

Example:

* Public path: `./build/`
* Script tag: ``
* Sourcemap annotation: `//# sourceMappingURL=./build/out.js.map`
* Resolved sourcemap location: `./build/build/out.js.map`

The correct annotation would be `//# sourceMappingURL=out.js.map` because sourcemaps are relative to the script tag `src`. The same issue happens if publicPath is `build` or `/build`.

I think the correct behavior is to prepend the publicPath only if it is absolute.

Sourcemap spec: https://github.com/tc39/source-map-spec/blob/main/source-map-rev3.md

> When the source mapping URL is not absolute, then it is relative to the generated code’s “source origin”. The source origin is determined by one of the following cases:
> * If the generated source is not associated with a script element that has a “src” attribute and there exists a //# sourceURL comment in the generated code, that comment should be used to determine the source origin. Note: Previously, this was “//@ sourceURL”, as with “//@ sourceMappingURL”, it is reasonable to accept both but //# is preferred.
> * If the generated code is associated with a script element and the script element has a “src” attribute, the “src” attribute of the script element will be the source origin.
> * If the generated code is associated with a script element and the script element does not have a “src” attribute, then the source origin will be the page’s origin.
> * If the generated code is being evaluated as a string with the eval() function or via new Function(), then the source origin will be the page’s origin.

Related issues:
* Same issue reported here: https://github.com/rails/sprockets-rails/pull/501
* This closed issue is about adding an API to allow arbitrary sourcemap prefixes, but the last comment complains about a similar issue but using entryNames instead of publicPath. I could not reproduce their issue though. https://github.com/evanw/esbuild/issues/2254#issuecomment-1956690026.

Start with the publicPath and linked sourcemap annotation behavior described in the example, then consult the referenced source map specification. Trace how relative and absolute publicPath values produce the annotation; done means a relative annotation resolves relative to the script src while an absolute publicPath is still prepended.

Written by the indexing model from the issue text.