cmd/go: integrate changes made in x/mod packages into internal packages

This change integrates changes made to x/mod packages into our internal
copies of those packages.

This is the first step of a bidirectional synchronization. A follow-up
change will copy changes made to the internal packages after x/mod was
forked. After that, we can vendor x/mod, update imports, and delete
the internal copies.

The following packages are affected:

* internal/module
* internal/semver (no change)
* internal/sumweb (renamed to internal/sumdb)
* internal/dirhash
* internal/note
* internal/tlog

Several integrated changes affect other packages:

* cmd/go/internal/module.MatchPathMajor now wraps a new function,
  CheckPathMajor, which returns error. MatchPathMajor returns
  bool. This will avoid an incompatible change in the next step.
* module.EncodePath renamed to EscapePath, EncodeVersion to
  EscapeVersion, DecodePath to UnescapePath, DecodeVersion to
  UnescapeVersion.
* cmd/go/internal/sumweb moved to cmd/go/internal/sumdb and package
  renamed to sumdb.
* sumdb.Client renamed to ClientOps, Conn to Client, Server to
  ServerOps, Paths to ServerPaths.
* sumdb/encode.go and encode_test.go are not present in x/mod since
  they are redundant with functionality in module. Both files are
  deleted.
* sumdb.TestServer doesn't implement sumdb.ServerOps after changes
  were were made to golang.org/x/mod/sumdb.ServerOps during the fork.
  Local changes made so tests will pass. These will be copied to x/mod
  in the next step.

Updates #34801

Change-Id: I7e820f10ae0cdbec238e59d039e978fd1cdc7201
Reviewed-on: https://go-review.googlesource.com/c/go/+/200138


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>