vendor: migrate from govendor to go mod vendor

The rsc.io/pdf package was added to the vendor directory 5 years ago
in CL 13968, back when the vendor directory was a part of the Go 1.5
vendor experiment.

The vendor.json file was written by hand in a format compatible with
the govendor tool. By now, that tool has been deprecated in favor of
the go command in module mode.

A go.mod file requiring a newer version of rsc.io/pdf was added in
CL 167137. Modify the vendor directory to use the newer rsc.io/pdf
version by recreating vendor directory using go command in Go 1.14:

	rm -rf vendor
	go mod vendor

It's possible the vendor directory isn't needed anymore and can be
safely deleted in a future change. The scope of this CL is only to
migrate from the deprecated govendor tool to the supported go tool;
deleting the vendor directory can still be done in a future change.

Remove the explicit mention of Go 1.5 vendor experiment from README
since it's old, and not relevant by now. Add a contributing section.

Updates golang/go#30228
Updates golang/go#33848

Change-Id: I95de95a3b2d81faf7235c675e5b8d425141f8d7a
Reviewed-on: https://go-review.googlesource.com/c/arch/+/222538
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

README.md

@@ -3,4 +3,11 @@
 This repository holds machine architecture information used by the Go toolchain.
 The parts needed in the main Go repository are copied in.
 
-This repository requires Go 1.5+ with the vendor experiment enabled.
+## Report Issues / Send Patches
+
+This repository uses Gerrit for code changes. To learn how to submit changes to
+this repository, see https://golang.org/doc/contribute.html.
+
+The main issue tracker for the arch repository is located at
+https://github.com/golang/go/issues. Prefix your issue with "x/arch:" in the
+subject line, so it is easy to find.

vendor/modules.txt

@@ -0,0 +1,2 @@
+# rsc.io/pdf v0.1.1
+rsc.io/pdf