[dev.regabi] all: merge master (ff0e93ea) into dev.regabi

Conflicts:

- src/cmd/compile/internal/gc/walk.go

  gc/walk.go is changed in CL 290950 on the master branch but
  deleted in the dev.regabi branch and moved over to the walk
  package. This CL includes porting CL 290950 over to the new
  walk.

Merge List:

+ 2021-02-12 ff0e93ea doc/go1.16: note that package path elements beginning with '.' are disallowed
+ 2021-02-11 249da7ec CONTRIBUTORS: update for the Go 1.16 release
+ 2021-02-11 864d4f1c cmd/go: multiple small 'go help' fixes
+ 2021-02-11 26ceae85 spec: More precise wording in section on function calls.
+ 2021-02-11 930c2c9a cmd/go: reject embedded files that can't be packed into modules
+ 2021-02-11 e5b08e6d io/fs: allow backslash in ValidPath, reject in os.DirFS.Open
+ 2021-02-10 ed807909 cmd/compile: mark concrete call of reflect.(*rtype).Method as REFLECTMETHOD
+ 2021-02-09 e9c96835 cmd/go: suppress errors from 'go get -d' for packages that only conditionally exist
+ 2021-02-09 e0ac989c archive/tar: detect out of bounds accesses in PAX records resulting from padded lengths
+ 2021-02-09 c9d6f45f runtime/metrics: fix a couple of documentation typpos
+ 2021-02-09 cea4e21b io/fs: backslash is always a glob meta character
+ 2021-02-08 dc725bfb doc/go1.16: mention new vet check for asn1.Unmarshal

Change-Id: Ib28fffa7dfbff7f6cdbfaf4a304757fead7bbf19