cmd/go/internal/get: move wildcard-trimming to before CheckImportPath

Previously, RepoRootForImportPath trimmed certain "..." wildcards from
package patterns (even though its name suggests that the argument must
be an actual import path). It trimmed at the first path element that
was literally "..." (although wildcards in general may appear within a
larger path element), and relied on a subsequent check in
RepoRootForImportPath to catch confusing resolutions.

However, that causes 'go get' with wildcard patterns in fresh paths to
fail as of CL 154101: a wildcard pattern is not a valid import path,
and fails the path check. (The existing Test{Vendor,Go}Get* packages
in go_test.go and vendor_test.go catch the failure, but they are all
skipped when the "-short" flag is set — including in all.bash — and we
had forgotten to run them separately.)

We now trim the path before any element that contains a wildcard, and
perform the path check (and repo resolution) on only that prefix. It
is possible that the expanded path after fetching the repo will be
invalid, but a repository can contain directories that are not valid
import paths in general anyway.

Fixes #29241

Change-Id: I70fb2f7fc6603b7d339fd6c02e8cdeacfc93fc4b
Reviewed-on: https://go-review.googlesource.com/c/154108


Reviewed-by: Russ Cox <rsc@golang.org>