[Bug] Source maps are broken when used with SrcOptions#base option

[chalkygames123]

This plugin breaks source maps when used with the base option of gulp.src(), and Chrome DevTools, for example, shows an error in the Sources panel like this:

Could not load content for http://localhost:8080/dist/styles/_partial.scss (HTTP error: status code 404, net::ERR_HTTP_RESPONSE_CODE_FAILURE)

Also, even without the base option, when using gulp's built-in source maps support, only the source maps for partials will be broken, and the source maps for stylesheets in node_modules will work.

Quick summary:

Using the base option?	Source Maps provider	Are the source maps for partials valid?	Are the source maps for node_modules valid?
No	Plugin	Yes	Yes
Yes	Plugin	No	No
No	Built-in	No	Yes
Yes	Built-in	No	No

Minimal reproduction: https://github.com/chalkygames123/repro-mr-hope__gulp-sass-source-maps

As a workaround, enabling sourceMapIncludeSources option of the Sass compiler fixes this issue.

[Mister-Hope]

I appreciate the detailed information, I am not having much time to check this these days, so I would appreciate it if you could open a pr to fix it.

The problem is that the code is written too old for me and I do not have much memory for gulp-sourcemap or related source map conversion codes