cmd: pull in golang.org/x/mod@3a5865c

This change updates the cmd module's requirement on x/mod and vendors in
the changes.

This pulls in the following changes into our vendored copy of x/mod:
golang.org/cl/351319: module: accept trailing slash in MatchPrefixPattern
golang.org/cl/353749: semver: remove unused err field
golang.org/cl/355630: x/mod: update requirement on x/crypto
golang.org/cl/359412: modfile: rename directory directive to use

Changes have been made in cmd/go renaming all uses of directory to use
and fixing references to functions in x/mod/modfile to account for the
changes in the last of thse CLs.

For #45713

Change-Id: I9121d08f6e6b11838bca50e6cbd756baeeae867b
Reviewed-on: https://go-review.googlesource.com/c/go/+/364114


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>