cue/load: deduplicate handling missing files for ad-hoc packages

When loading a list of CUE files as a "files package",
we would first stat the files to fail early if they were not
present and regular files.

However, this is unnecessary; we parse the files immediately afterwards,
a process which already fails with the standard OS errors when one tries
to open and read a missing file or a directory.

This saves 2226 syscalls when running `go test ./...`,
as there are many Go tests which use ad-hoc packages.
For example, all the tests using cuetxtar with txtar archives.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I42497c70b46a77ffe2907d99c4ed8e9398e6c8be
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1218101
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

Author: mvdan

cmd/cue/cmd/testdata/script/get_crd_errors.txtar

@@ -4,9 +4,9 @@
 ! exec cue get crd
 stderr 'must specify at least one file'
 
-# Test non-existent file
+# Test non-existent file; different errors on unix vs windows
 ! exec cue get crd nonexistent.yaml
-stderr 'could not find file'
+stderr 'no such file or directory|cannot find the file specified'
 
 # Test invalid YAML
 ! exec cue get crd invalid.yaml

cue/load/loader.go

@@ -81,23 +81,6 @@ func (l *loader) cueFilesPackage(files []*build.File) *build.Instance {
 	// ModInit() // TODO: support modules
 	pkg := l.cfg.Context.NewInstance(l.cfg.Dir, nil)
 
-	for _, bf := range files {
-		f := bf.Filename
-		if f == "-" {
-			continue
-		}
-		if !filepath.IsAbs(f) {
-			f = filepath.Join(l.cfg.Dir, f)
-		}
-		fi, err := l.cfg.fileSystem.stat(f)
-		if err != nil {
-			return l.cfg.newErrInstance(errors.Wrapf(err, token.NoPos, "could not find file %v", f))
-		}
-		if fi.IsDir() {
-			return l.cfg.newErrInstance(errors.Newf(token.NoPos, "file is a directory %v", f))
-		}
-	}
-
 	fp := newFileProcessor(l.cfg, pkg, l.tagger)
 	if l.cfg.Package == "*" {
 		fp.allPackages = true