cue/load: set the absolute file name when setting source

This means that when the file is decoded, the `internal/encoding.NewDecoder`
logic can find it, even though `internal/encoding.Config` doesn't
contain a directory field to interpret relative file paths.

This fixes a test failure that happens when the modules experiment
flag is enabled by default.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Ia606ce2dc0fca816f3986c24fda96041862b0297
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194830
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cue/load/import.go

@@ -250,6 +250,8 @@ func setFileSource(cfg *Config, f *build.File) error {
 	}
 	if !filepath.IsAbs(fullPath) {
 		fullPath = filepath.Join(cfg.Dir, fullPath)
+		// Ensure that encoding.NewDecoder will work correctly.
+		f.Filename = fullPath
 	}
 	if fi := cfg.fileSystem.getOverlay(fullPath); fi != nil {
 		if fi.file != nil {