build: load package in LoadAllSyntax mode (#63)

Fixes #62

tests/fixedbugs/issue62/asm.go

@@ -0,0 +1,14 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+)
+
+func main() {
+	Package("github.com/mmcloughlin/avo/tests/fixedbugs/issue62")
+	Implement("private")
+	RET()
+	Generate()
+}

tests/fixedbugs/issue62/issue62.s

@@ -0,0 +1,5 @@
+// Code generated by command: go run asm.go -out issue62.s. DO NOT EDIT.
+
+// func private()
+TEXT ·private(SB), $0
+	RET

tests/fixedbugs/issue62/issue62_test.go

@@ -0,0 +1,11 @@
+package issue62
+
+import (
+	"testing"
+)
+
+//go:generate go run asm.go -out issue62.s
+
+func TestPrivate(t *testing.T) {
+	private()
+}

tests/fixedbugs/issue62/stub.go

@@ -0,0 +1,5 @@
+// Package issue62 tests for using Implement() with an unexported function.
+package issue62
+
+// private serves as an example unexported function to implement.
+func private()