De-lint 'tftp-go'

[tstromberg]

This PR does 3 things:

• Adopts Go Modules: https://go.dev/blog/using-go-modules
• Adds lint-configuration from http://github.com/tinkerbell/lint-install
• Addresses Go lint errors:

handler.go:115:71: (*session).writeAndWaitForPacket - result 0 (github.com/tinkerbell/tftp-go.packet) is never used (unparam)
func (s *session) writeAndWaitForPacket(p packet, v packetValidator) (packet, error) {
                                                                      ^
handler.go:130:7: comparing with == will fail on wrapped errors. Use errors.Is to check for a specific error (errorlint)
			if err == ErrTimeout {
			   ^
handler.go:182:3: ifElseChain: rewrite if-else to switch statement (gocritic)
		if i < 8 {
		^
handler.go:201:3: ifElseChain: rewrite if-else to switch statement (gocritic)
		if i < 1 {
		^
handler.go:230:3: switch on an error will fail on wrapped errors. Use errors.Is to check for specific errors (errorlint)
		switch err {
		^
handler.go:280:3: switch on an error will fail on wrapped errors. Use errors.Is to check for specific errors (errorlint)
		switch readErr {
		^
handler.go:303:28: unused-parameter: parameter 'p' seems to be unused, consider removing or renaming it as _ (revive)
func (s *session) serveWRQ(p *packetWRQ) {
                           ^
handler_test.go:69:29: Comment should end in a period (godot)
// To implement packetReader
                            ^
handler_test.go:70:31: unused-parameter: parameter 'timeout' seems to be unused, consider removing or renaming it as _ (revive)
func (h *handlerContext) read(timeout time.Duration) (packet, error) {
                              ^
handler_test.go:86:26: Comment should end in a period (godot)
// Implement packetWriter
                         ^
handler_test.go:92:24: Comment should end in a period (godot)
// To implement Handler
                       ^
handler_test.go:100:24: Comment should end in a period (godot)
// To implement Handler
                       ^
handler_test.go:120:26: test helper function should start from t.Helper() (thelper)
func (h *handlerContext) Negotiate(t *testing.T, o map[string]string) {
                         ^
handler_test.go:126:2: type assertion must be checked (forcetypeassert)
	oack := poack.(*packetOACK)
	^
handler_test.go:144:2: type assertion must be checked (forcetypeassert)
	p := px.(*packetERROR)
	^
handler_test.go:156:2: type assertion must be checked (forcetypeassert)
	p := px.(*packetERROR)
	^
handler_test.go:200:3: type assertion must be checked (forcetypeassert)
		p := px.(*packetERROR)
		^
handler_test.go:317: File is not `gofmt`-ed with `-s` (gofmt)
				&packetDATA{blockNr: 1, data: []byte{0x0, 0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7}},
				&packetDATA{blockNr: 2, data: []byte{0x8, 0x9, 0xa, 0xb, 0xc, 0xd, 0xe, 0xf}},
				&packetDATA{blockNr: 3, data: []byte{}},
handler_test.go:341:4: type assertion must be checked (forcetypeassert)
			actual := pdata.(*packetDATA)
			^
handler_test.go:370:2: type assertion must be checked (forcetypeassert)
	data := pdata.(*packetDATA)
	^
packet.go:54:2: `tftpErrDiskFull` is unused (deadcode)
	tftpErrDiskFull          = tftpError{3, "Disk full or allocation exceeded."}
	^
packet.go:56:2: `tftpErrUnknownTransferID` is unused (deadcode)
	tftpErrUnknownTransferID = tftpError{5, "Unknown transfer ID."}
	^
packet.go:57:2: `tftpErrFileAlreadyExists` is unused (deadcode)
	tftpErrFileAlreadyExists = tftpError{6, "File already exists."}
	^
packet.go:58:2: `tftpErrNoSuchUser` is unused (deadcode)
	tftpErrNoSuchUser        = tftpError{7, "No such user."}
	^
packet.go:90:5: variable 'err' is only used in the if-statement (packet.go:91:2); consider using short syntax (ifshort)
	_, err := b.WriteString(line)
	   ^
packet.go:161:31: unnecessary conversion (unconvert)
	if err = writeChunk(b, string(p.filename)); err != nil {
	                             ^
packet.go:167:2: if-return: redundant if ...; err != nil check, just return error instead. (revive)
	if err = writeOptions(b, p.options); err != nil {
		return err
	}
packet.go:321:2: if-return: redundant if ...; err != nil check, just return error instead. (revive)
	if err = p.Write(b); err != nil {
		return err
	}
packet_test.go:49:6: test helper function should start from t.Helper() (thelper)
func testReadPacketXRQ(t *testing.T, prefix []byte) {
     ^
packet_test.go:207:3: type assertion must be checked (forcetypeassert)
		px := p.(*packetDATA)
		^
packet_test.go:248:3: type assertion must be checked (forcetypeassert)
		px := p.(*packetACK)
		^
packet_test.go:298:3: type assertion must be checked (forcetypeassert)
		px := p.(*packetERROR)
		^
packet_test.go:312:3: type assertion must be checked (forcetypeassert)
		px := p.(*packetERROR)
		^
packet_test.go:372:3: type assertion must be checked (forcetypeassert)
		px := p.(*packetOACK)
		^