tweak when we read and write cachedOutput files

We first called the typecheck method, which starts filling cachedOutput
with information from the current package, and later we would load the
gob files for all dependencies via loadCachedOutputs.

This was a bit confusing; instead, load the cached gob files first,
and then do all the operations which fill information for curPkg.

Similarly, we were waiting until the very end of transformCompile to
write curPkg's cachedOutput gob file to the disk cache.
We can write the file at an earlier point, before we have obfuscated and
re-printed all Go files for the current package.
We can also write the file before other work like processImportCfg.

None of these changes should affect garble's behavior,
but they will make the cache redesign for #708 easier.

main.go

@@ -894,40 +894,57 @@ func transformCompile(args []string) ([]string, error) {
 		files = append(files, file)
 	}
 	tf := newTransformer()
-	if err := tf.typecheck(files); err != nil {
-		return nil, err
-	}
-
-	flags = alterTrimpath(flags)
 
+	// cachedOutput is modified starting at this point, with the typecheck call.
+	// We use an extra syntax block to clarify what bits of code set up the caching.
 	// Note that if the file already exists in the cache from another build,
 	// we don't need to write to it again thanks to the hash.
 	// TODO: as an optimization, just load that one gob file.
-	if err := loadCachedOutputs(); err != nil {
-		return nil, err
-	}
+	{
+		if err := loadCachedOutputs(); err != nil {
+			return nil, err
+		}
+
+		if err := tf.typecheck(files); err != nil {
+			return nil, err
+		}
 
-	ssaProg := ssa.NewProgram(fset, 0)
+		ssaProg := ssa.NewProgram(fset, 0)
 
-	// Create SSA packages for all imports.
-	// Order is not significant.
-	created := make(map[*types.Package]bool)
-	var createAll func(pkgs []*types.Package)
-	createAll = func(pkgs []*types.Package) {
-		for _, p := range pkgs {
-			if !created[p] {
-				created[p] = true
-				ssaProg.CreatePackage(p, nil, nil, true)
-				createAll(p.Imports())
+		// Create SSA packages for all imports.
+		// Order is not significant.
+		created := make(map[*types.Package]bool)
+		var createAll func(pkgs []*types.Package)
+		createAll = func(pkgs []*types.Package) {
+			for _, p := range pkgs {
+				if !created[p] {
+					created[p] = true
+					ssaProg.CreatePackage(p, nil, nil, true)
+					createAll(p.Imports())
+				}
 			}
 		}
-	}
-	createAll(tf.pkg.Imports())
+		createAll(tf.pkg.Imports())
+
+		ssaPkg := ssaProg.CreatePackage(tf.pkg, files, tf.info, false)
+		ssaPkg.Build()
 
-	ssaPkg := ssaProg.CreatePackage(tf.pkg, files, tf.info, false)
-	ssaPkg.Build()
+		tf.recordReflection(ssaPkg)
 
-	tf.recordReflection(ssaPkg)
+		if err := tf.prefillObjectMaps(files); err != nil {
+			return nil, err
+		}
+
+		if err := writeGobExclusive(
+			garbleExportFile(curPkg),
+			cachedOutput,
+		); err != nil && !errors.Is(err, fs.ErrExist) {
+			return nil, err
+		}
+	}
+	// cachedOutput isn't modified after this point.
+
+	flags = alterTrimpath(flags)
 	newImportCfg, err := processImportCfg(flags)
 	if err != nil {
 		return nil, err
@@ -941,10 +958,6 @@ func transformCompile(args []string) ([]string, error) {
 	// log.Printf("seeding math/rand with %x\n", randSeed)
 	obfRand = mathrand.New(mathrand.NewSource(int64(binary.BigEndian.Uint64(randSeed))))
 
-	if err := tf.prefillObjectMaps(files); err != nil {
-		return nil, err
-	}
-
 	// If this is a package to obfuscate, swap the -p flag with the new package path.
 	// We don't if it's the main package, as that just uses "-p main".
 	// We only set newPkgPath if we're obfuscating the import path,
@@ -1016,13 +1029,6 @@ func transformCompile(args []string) ([]string, error) {
 	}
 	flags = flagSetValue(flags, "-importcfg", newImportCfg)
 
-	if err := writeGobExclusive(
-		garbleExportFile(curPkg),
-		cachedOutput,
-	); err != nil && !errors.Is(err, fs.ErrExist) {
-		return nil, err
-	}
-
 	return append(flags, newPaths...), nil
 }