cmd/cue: load errors on multiple packages are different with a trailing slash

[mvdan]

exec cue vet -c ./multi
exec cue vet -c ./multi/

-- multi/multi.cue --
package multi
multi: "x"
-- multi/foo.cue --
package foo
foo: "x"
-- multi/bar.cue --
package bar
bar: "x"
-- multi/baz.cue --
package baz
baz: "x"

As of 870bfc2, I see:

> exec cue vet -c ./multi
[stderr]
found packages "bar" (bar.cue) and "baz" (baz.cue) in "$WORK/multi"
found packages "bar" (bar.cue) and "foo" (foo.cue) in "$WORK/multi"
found packages "bar" (bar.cue) and "multi" (multi.cue) in "$WORK/multi"
[exit status 1]
FAIL: repro-cmd.txtar:1: unexpected command failure
> exec cue vet -c ./multi/
[stderr]
found packages "bar" (bar.cue) and "baz" (baz.cue) in "multi"
[exit status 1]

It's not clear to me why the first prints three errors, but the second prints just one. Also odd that the path is absolute in only the first case.