resolve-url is only needed by browsers. This allows using npm as a
package manager for browser projects too. Node.js users shouln’t be
bothered since resolve-url is so small; the extra download shouldn’t
A source root such as `/scripts/subdir` is not treated as
`/scripts/subdir/` — that is, as a directory called “subdir”, not a file
called “subdir”. Pointing to a file as source root does not makes sense.