cmd/go: have go mod vendor copy embedded files in subdirs

If a package vendored with go mod vendor depends on embedded
files contained in subdirectories, copy them into the the
corresponding place in the module's vendor tree. (Embeds in
parent directories are disallowed by the embed pattern rules, and
embeds in the same directory are copied because go mod vendor
already copies the non-go files in the package's own directory).

Export the vendor pattern expansion code in internal/load so
internal/modcmd's vendor code can use it.

Fixes #43077

Change-Id: I61edb344d73df590574a6498ffb6069e8d72a147
Reviewed-on: https://go-review.googlesource.com/c/go/+/283641


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