cmd/gomobile: detect missing golang.org/x/mobile dep before invoking gobind

gobind's generated code depends on packages under golang.org/x/mobile/bind,
and gobind resolves them through the calling module via packages.Load. When
the dependency is missing from the user's module — most commonly after
go mod tidy removes it because no non-generated code imports it — gobind
fails with an opaque message repeated once per sub-target:

    unable to import bind: no Go package in golang.org/x/mobile/bind
    unable to import bind: no Go package in golang.org/x/mobile/bind
    unable to import bind/objc: no Go package in golang.org/x/mobile/bind/objc

Detect the missing module dependency in cmd/gomobile before invoking gobind
and emit a single actionable message that suggests adding a tool directive
to go.mod via 'go get -tool golang.org/x/mobile/cmd/gobind'.

Fixes golang/go#77183.

Change-Id: I365c99a1f37bec1f42b6cb858babd33d41317a8a
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/779840
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Hajime Hoshi <hajimehoshi@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Hajime Hoshi <hajimehoshi@gmail.com>