build: -pkg flag to specify package name (#150)

Allows override in the case where package name is incorrectly deduced from directory name.

Fixes #68
Fixes #147

tests/fixedbugs/issue68/asm.go

@@ -0,0 +1,18 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/operand"
+)
+
+func main() {
+	TEXT("Issue68", NOSPLIT, "func() uint64")
+	Doc("Issue68 tests custom package names.")
+	x := GP64()
+	MOVQ(U32(68), x)
+	Store(x, ReturnIndex(0))
+	RET()
+	Generate()
+}

tests/fixedbugs/issue68/doc.go

@@ -0,0 +1,2 @@
+// Package custom tests overriding package name with the CLI.
+package custom

tests/fixedbugs/issue68/issue68_test.go

@@ -0,0 +1,13 @@
+package custom
+
+import (
+	"testing"
+)
+
+//go:generate go run asm.go -pkg custom -out issue69.s -stubs stub.go
+
+func TestIssue68(t *testing.T) {
+	if got := Issue68(); got != 68 {
+		t.Fail()
+	}
+}

tests/fixedbugs/issue68/issue69.s

@@ -0,0 +1,9 @@
+// Code generated by command: go run asm.go -pkg custom -out issue69.s -stubs stub.go. DO NOT EDIT.
+
+#include "textflag.h"
+
+// func Issue68() uint64
+TEXT ·Issue68(SB), NOSPLIT, $0-8
+	MOVQ $0x00000044, AX
+	MOVQ AX, ret+0(FP)
+	RET

tests/fixedbugs/issue68/stub.go

@@ -0,0 +1,6 @@
+// Code generated by command: go run asm.go -pkg custom -out issue69.s -stubs stub.go. DO NOT EDIT.
+
+package custom
+
+// Issue68 tests custom package names.
+func Issue68() uint64