cmd/go/internal/get: allow go get on github.com/ import paths with Unicode letters

cmd/go/internal/get: allow go get on github.com/ import paths with Unicode letters

More specifically, allow Unicode letters in the directories of GitHub
repositories, which can occur and don't have a valid reason to be
disallowed by go get.

Do so by using a predefined character class, the Unicode character
property class \p{L} that describes the Unicode characters that are
letters:

	http://www.regular-expressions.info/unicode.html#category

Since it's not possible to create GitHub usernames or repositories
containing Unicode letters at this time, those parts of the import path
are still restricted to ASCII letters only.

Fix name of tested func in t.Errorf messages.

Fixes #18660.

Change-Id: Ia0ef4742bfd8317d989ef1eb1d7065e382852fe2
Reviewed-on: https://go-review.googlesource.com/41822
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>