pkg/urlutil (despite its poorly chosen name) is not really intended as a
generic utility to handle URLs, and should only be used by the builder to
handle (remote) build contexts.
The `IsURL()` function only does a very rudimentary check for `http(s)://`
prefixes, without any other validation, but due to its name may give
incorrect expectations.
As we're deprecating this package for uses other than for build-contexts,
this patch replaces this instance of the utility for a local function.
While changing, also cleaned up some intermediate variables, and made
the logic slightly more descriptive.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
052b4086b9