Option to disable loading of existing source maps for the source

The old `gulp-sourcemaps` had a `loadMaps` setting to control whether existing maps should be loaded. Gulp 4 has a single setting in `gulp.src` which controls *both* enabling sourcemaps and loading existing ones.

This cause issues for me when npm packages ship their bundle files with a sourcemap while they don't ship their original source code at the same time (and so they have sourcemaps pointing to non-existent files). For such cases, I'd rather have my own source maps point me to their bundle file (that I know about) rather than to their source code (that I don't know about).
I would be great if this feature could be available again.

**What version of gulp are you using?**
4.0.2

**What versions of npm and node are you using?**
node 12.14.1
yarn 1.21.1