This test demonstrates diagnostics for various forms of file exclusion.

Note: this test used to also check the errors when a file was excluded due to
an inactive module, or mismatching GOOS/GOARCH, comment, but with zero-config
gopls (golang/go#57979) and improved build tag support (golang/go#29202), we no
longer get these errors.

-- go.work --
go 1.21

use (
	./a
)
-- a/go.mod --
module mod.com/a

go 1.18

-- a/a.go --
package a

-- a/a_plan9.go --
package a // Not excluded, due to improved build tag support.

-- a/a_ignored.go --
//go:build skip
package a //@diag(re"package (a)", re"excluded due to its build tags")

-- b/go.mod --
module mod.com/b

go 1.18

-- b/b.go --
package b // Not excluded, due to zero-config gopls.