As it's just an alias for filepath.IsAbs. Also added a normalize step in
TrimBuildFilesFromExcludes, so that callers are not _required_ to first
normalize the path.
We are considering deprecating and/or removing this function in the archive
package, so removing it in the cli code helps transitioning if we decide to
deprecate and/or remove it.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>