dist.dependency_info(): return just relevant dependencies
increase cache timeout to 3hrs (was 1)
remove unnecessary line breaks
more accurate error messages
remove todo
handle the 'scm' version differently in 'tree' functionality

When the package has 'scm' version, then this is stored in
dependency manifest like 'pkg.was_scm_version = true' and
when printing the dependency information to the user, the
'scm' is printed instead of the version from the original
dist.info file.
delete the cached dependency manifest on 'refresh' funcionality
update help for 'tree'