For golang/go#18660 and golang/gddo#468.

Will be deleted after those issues are resolved.

Background

The following is a valid vanity import path that works without issues:

	dmitri.shuralyov.com/temp/go-get-issue-unicode/испытание

You can go get, go install, go test, go doc it without issues:

	$ go get -u dmitri.shuralyov.com/temp/go-get-issue-unicode/испытание
	$ go install dmitri.shuralyov.com/temp/go-get-issue-unicode/испытание
	$ go test dmitri.shuralyov.com/temp/go-get-issue-unicode/испытание
	ok  	dmitri.shuralyov.com/temp/go-get-issue-unicode/испытание	0.014s
	$ go doc dmitri.shuralyov.com/temp/go-get-issue-unicode/испытание
	package испытание // import "dmitri.shuralyov.com/temp/go-get-issue-unicode/испытание"

	Package испытание demonstrates Unicode capabilities in Go source code.

	type Эксперимент struct{ ... }
	    func Испытание() Эксперимент

You can also call vcs.RepoRootForImportPath("dmitri.shuralyov.com/temp/go-get-issue-unicode/испытание", false)
(from golang.org/x/tools/go/vcs) successfully on the vanity import path:

	$ goexec 'vcs.RepoRootForImportPath("dmitri.shuralyov.com/temp/go-get-issue-unicode/испытание", false)'
	(*vcs.RepoRoot)(&vcs.RepoRoot{
		VCS: (*vcs.Cmd)(&vcs.Cmd{
			Name:        (string)("Git"),
			Cmd:         (string)("git"),
			CreateCmd:   (string)("clone {repo} {dir}"),
			DownloadCmd: (string)("pull --ff-only"),
			TagCmd: ([]vcs.TagCmd)([]vcs.TagCmd{
				(vcs.TagCmd)(vcs.TagCmd{
					Cmd:     (string)("show-ref"),
					Pattern: (string)("(?:tags|origin)/(\\S+)$"),
				}),
			}),
			TagLookupCmd: ([]vcs.TagCmd)([]vcs.TagCmd{
				(vcs.TagCmd)(vcs.TagCmd{
					Cmd:     (string)("show-ref tags/{tag} origin/{tag}"),
					Pattern: (string)("((?:tags|origin)/\\S+)$"),
				}),
			}),
			TagSyncCmd:     (string)("checkout {tag}"),
			TagSyncDefault: (string)("checkout master"),
			LogCmd:         (string)(""),
			Scheme: ([]string)([]string{
				(string)("git"),
				(string)("https"),
				(string)("http"),
				(string)("git+ssh"),
			}),
			PingCmd: (string)("ls-remote {scheme}://{repo}"),
		}),
		Repo: (string)("https://github.com/shurcooL-test/go-get-issue-unicode"),
		Root: (string)("dmitri.shuralyov.com/temp/go-get-issue-unicode"),
	})
	(interface{})(nil)

However, gosrc.IsValidRemotePath incorrectly reports false for the
"dmitri.shuralyov.com/temp/go-get-issue-unicode/испытание" import path.

Fix

gosrc.IsValidRemotePath reports false for such import paths because
validPathElement regexp only allows ASCII letters A-Za-z, not Unicode
ones.

This change fixes that by using a predefined character class, the
Unicode character property class \p{L} that describes the Unicode
characters that are letters.

Additionally, fix an issue where a query parameter value was not
correctly escaped when constructing a URL.

Fixes #468.
Updates golang/go#18660.

References

-	https://stackoverflow.com/questions/3617797/regex-to-match-only-letters
-	https://stackoverflow.com/questions/6005459/is-there-a-way-to-match-any-unicode-non-alphabetic-character
-	https://www.regular-expressions.info/unicode.html#prop

Change-Id: I48680749d827cbc63fefca2c21e9790009f20746
Reviewed-on: https://go-review.googlesource.com/41750
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Tuo Shan <shantuo@google.com>
Reviewed-by: Francesc Campoy Flores <campoy@golang.org>

Manually apply same change as CL 41822 did for cmd/go/internal/get,
but for golang.org/x/tools/go/vcs, to help keep them in sync.

Updates golang/go#18660.
Helps golang/go#11490.

Change-Id: I6c7759c073583dea771bc438b70f8c2eb7b5ebfb
Reviewed-on: https://go-review.googlesource.com/42017
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>