Function is_git_source 

pub fn is_git_source(source: &str) -> bool

Check whether a file source string is a git URL (not a local path).

file:// URLs are rejected by default to keep remote-module sources to proper network protocols. Tests can opt into local-file sources by setting CFGD_ALLOW_LOCAL_SOURCES=1 — same gate sources/mod.rs uses for the composed-sources path.