Read an xz release in Go, so the build seat needs no xz tool (hq issue 345)

Betterbird's pinned .tar.xz failed on the build seat with "this build seat has
no xz to read it": the seat's image is the catalogue's build-agent recipe,
which installs git and docker-cli alone, and xz had been added only to
cmd/mesh-builder/Dockerfile, which the seat does not build from. The stream
is now read with github.com/ulikunitz/xz (vendored), its dictionary capped
at xz's own largest preset, and what it unpacks to bounded as before; the
xz line in the unused Dockerfile is taken back out.

vendor/github.com/ulikunitz/xz/lzma/literalcodec.go

@@ -0,0 +1,125 @@
+// Copyright 2014-2022 Ulrich Kunitz. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package lzma
+
+// literalCodec supports the encoding of literal. It provides 768 probability
+// values per literal state. The upper 512 probabilities are used with the
+// context of a match bit.
+type literalCodec struct {
+	probs []prob
+}
+
+// deepcopy initializes literal codec c as a deep copy of the source.
+func (c *literalCodec) deepcopy(src *literalCodec) {
+	if c == src {
+		return
+	}
+	c.probs = make([]prob, len(src.probs))
+	copy(c.probs, src.probs)
+}
+
+// init initializes the literal codec.
+func (c *literalCodec) init(lc, lp int) {
+	switch {
+	case !(minLC <= lc && lc <= maxLC):
+		panic("lc out of range")
+	case !(minLP <= lp && lp <= maxLP):
+		panic("lp out of range")
+	}
+	c.probs = make([]prob, 0x300<<uint(lc+lp))
+	for i := range c.probs {
+		c.probs[i] = probInit
+	}
+}
+
+// Encode encodes the byte s using a range encoder as well as the current LZMA
+// encoder state, a match byte and the literal state.
+func (c *literalCodec) Encode(e *rangeEncoder, s byte,
+	state uint32, match byte, litState uint32,
+) (err error) {
+	k := litState * 0x300
+	probs := c.probs[k : k+0x300]
+	symbol := uint32(1)
+	r := uint32(s)
+	if state >= 7 {
+		m := uint32(match)
+		for {
+			matchBit := (m >> 7) & 1
+			m <<= 1
+			bit := (r >> 7) & 1
+			r <<= 1
+			i := ((1 + matchBit) << 8) | symbol
+			if err = probs[i].Encode(e, bit); err != nil {
+				return
+			}
+			symbol = (symbol << 1) | bit
+			if matchBit != bit {
+				break
+			}
+			if symbol >= 0x100 {
+				break
+			}
+		}
+	}
+	for symbol < 0x100 {
+		bit := (r >> 7) & 1
+		r <<= 1
+		if err = probs[symbol].Encode(e, bit); err != nil {
+			return
+		}
+		symbol = (symbol << 1) | bit
+	}
+	return nil
+}
+
+// Decode decodes a literal byte using the range decoder as well as the LZMA
+// state, a match byte, and the literal state.
+func (c *literalCodec) Decode(d *rangeDecoder,
+	state uint32, match byte, litState uint32,
+) (s byte, err error) {
+	k := litState * 0x300
+	probs := c.probs[k : k+0x300]
+	symbol := uint32(1)
+	if state >= 7 {
+		m := uint32(match)
+		for {
+			matchBit := (m >> 7) & 1
+			m <<= 1
+			i := ((1 + matchBit) << 8) | symbol
+			bit, err := d.DecodeBit(&probs[i])
+			if err != nil {
+				return 0, err
+			}
+			symbol = (symbol << 1) | bit
+			if matchBit != bit {
+				break
+			}
+			if symbol >= 0x100 {
+				break
+			}
+		}
+	}
+	for symbol < 0x100 {
+		bit, err := d.DecodeBit(&probs[symbol])
+		if err != nil {
+			return 0, err
+		}
+		symbol = (symbol << 1) | bit
+	}
+	s = byte(symbol - 0x100)
+	return s, nil
+}
+
+// minLC and maxLC define the range for LC values.
+const (
+	minLC = 0
+	maxLC = 8
+)
+
+// minLC and maxLC define the range for LP values.
+const (
+	minLP = 0
+	maxLP = 4
+)