cue/load: treat stdlib-like import on command line as error

It's not currently possible to name a stdlib package on the command
line, except for the edge case of when a directory of that name exists
in cue.mod/{pkg,gen,usr}.

Close off that edge case by making it an error to import such a package
as part of the command-line arguments. In the future, we might want to
explicitly allow this, but the current situation seems wrong.

This also fixes stdlib-imports in modules mode, because it prevents the
resolution logic from ever seeing a package with no module.

Fixes #2964

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I22bbe764bb16fb2a992189981c9bb9d28fd3e78a
Dispatch-Trailer: {"type":"trybot","CL":1185363,"patchset":3,"ref":"refs/changes/63/1185363/3","targetBranch":"master"}

cmd/cue/cmd/testdata/script/eval_loaderr.txtar

@@ -3,4 +3,4 @@
 cmp stderr expect-stderr
 
 -- expect-stderr --
-cannot find package "nonexisting"
+"nonexisting" cannot be imported as CUE package

cmd/cue/cmd/testdata/script/issue2964.txtar

@@ -0,0 +1,6 @@
+env CUE_EXPERIMENT=modules
+exec cue mod init test.example
+! exec cue export anypath
+cmp stderr want-stderr
+-- want-stderr --
+"anypath" cannot be imported as CUE package

cue/load/import.go

@@ -213,7 +213,7 @@ func (l *loader) _loadFunc(pos token.Pos, path string) *build.Instance {
 		return l.cfg.newErrInstance(errors.Newf(pos, "relative import paths not allowed (%q)", path))
 	}
 
-	if isStdlibImport(path) {
+	if isStdlibPackage(path) {
 		// It looks like a builtin.
 		return nil
 	}
@@ -326,6 +326,9 @@ func (l *loader) absDirFromImportPath(pos token.Pos, p importPath) (absDir, name
 	if l.cfg.ModuleRoot == "" {
 		return "", "", errors.Newf(pos, "cannot import %q (root undefined)", p)
 	}
+	if isStdlibPackage(string(p)) {
+		return "", "", errors.Newf(pos, "%q cannot be imported as CUE package", p)
+	}
 	origp := p
 	// Extract the package name.
 	parts := module.ParseImportPath(string(p))
@@ -407,5 +410,5 @@ func absPathForSourceLoc(loc module.SourceLoc) (string, error) {
 // isStdlibPackage reports whether pkgPath looks like
 // an import from the standard library.
 func isStdlibPackage(pkgPath string) bool {
-	return strings.IndexByte(strings.Split(path, "/")[0], '.') == -1
+	return strings.IndexByte(strings.Split(pkgPath, "/")[0], '.') == -1
 }

cue/load/loader_test.go

@@ -222,24 +222,21 @@ display:foo.com/bad-identifier`,
 	}, {
 		cfg:  dirCfg,
 		args: []string{"nonexisting"},
-		want: `err:    cannot find package "nonexisting"
+		want: `err:    "nonexisting" cannot be imported as CUE package
 path:   nonexisting
 module: mod.test/test
 root:   $CWD/testdata/testmod
-dir:    $CWD/testdata/testmod/cue.mod/gen/nonexisting
+dir:    ""
 display:nonexisting`,
 	}, {
-		// TODO this should not work, as "strconv" is a standard library
-		// import and should always be treated as such.
 		cfg:  dirCfg,
 		args: []string{"strconv"},
-		want: `path:   strconv
+		want: `err:    "strconv" cannot be imported as CUE package
+path:   strconv
 module: mod.test/test
 root:   $CWD/testdata/testmod
-dir:    $CWD/testdata/testmod/cue.mod/gen/strconv
-display:strconv
-files:
-    $CWD/testdata/testmod/cue.mod/pkg/strconv/strconv.cue`,
+dir:    ""
+display:strconv`,
 	}, {
 		cfg:  dirCfg,
 		args: []string{"./empty"},