monopam/lint: check source URL consistency
For each subtree, compare the URL derived from dune-project's
(source (...)) stanza against sources.toml's source and upstream
fields. Flag the subtree when neither matches -- that usually means a
rename happened on one side and not the other, or the dune-project URL
was never updated after a fork.
Does not hit the network; URL normalisation strips git+ prefix,
trailing .git, and trailing slash before comparing. Added source_issue
type, extended Lint.result with source_issues, and the lint command
prints a second report block when any are present.