internal/mod/modpkgload: propagate CUE syntax error

When there's a syntax error reading CUE syntax, the
error details were being hidden. This resulted
in issue #3281. Using `%w` instead of `%v` fixes this.

Fixes #3281.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I43e49cde84319de9a20a395eea46664831e8a470
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1197984
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cmd/cue/cmd/testdata/script/issue3281.txtar

@@ -0,0 +1,16 @@
+# When there's a syntax error encountered at an early
+# stage of processing, the error message should
+# point to the place in the file where the error was
+# encountered.
+
+! exec cue eval invalid.cue
+stderr invalid.cue:3:
+-- cue.mod/module.cue --
+module: "test.example"
+language: version: "v0.9.0"
+
+-- invalid.cue --
+package blah
+
+self: bad )
+syntax

cue/load/instances.go

@@ -205,7 +205,7 @@ func loadPackages(
 		}
 		syntax, err := cfg.fileSystem.getCUESyntax(f)
 		if err != nil {
-			return nil, fmt.Errorf("cannot get syntax for %q: %v", f.Filename, err)
+			return nil, fmt.Errorf("cannot get syntax for %q: %w", f.Filename, err)
 		}
 		for _, imp := range syntax.Imports {
 			pkgPath, err := strconv.Unquote(imp.Path.Value)