Incorrect handling of path separators under Windows

[loewenheim]

Trying to inject debug IDs into sourcemaps in a different directory (i.e. not the current directory) fails on Windows. The reason is inconsistent handling of path separators. Specifically, at

sentry-cli/src/utils/sourcemaps.rs

Line 995 in 5e5949c

if self.sources.contains_key(&sourcemap_url) {

the sources contain URLs with slashes but sourcemap_url contains backslashes. This means that we don't find the sourcemap.