cliflagv2: read each flag from the command that defines it

[abo3losh1]

Follow-up to #451, same problem in the v2 provider.

processFlags reads every flag through p.ctx, the innermost command's context, and findFlag only looks at the app's flags and the innermost command's flags. With app > mid > leaf:

app := &cli.App{
	Name:  "app",
	Flags: []cli.Flag{&cli.StringFlag{Name: "name"}},
	Commands: []*cli.Command{{
		Name:  "mid",
		Flags: []cli.Flag{&cli.StringFlag{Name: "region"}, &cli.IntFlag{Name: "port"}},
		Subcommands: []*cli.Command{{
			Name:   "leaf",
			Flags:  []cli.Flag{&cli.StringFlag{Name: "name"}},
			Action: func(ctx *cli.Context) error { /* Provider(ctx, ".").Read() */ },
		}},
	}},
}
// app --name root mid --region eu --port 80 leaf --name child

master gives:

map[app:map[mid:map[leaf:map[name:child]] name:child]]

• mid.region and mid.port are missing, because findFlag never looks at mid.
• app.name is child, because p.ctx.String("name") finds the leaf's flag first.
• With only leaf --name child, app.name is still set to child.

Read() already walks the lineage, and each entry is that command's own *cli.Context. The fix passes that context to processFlags and uses the flag we already hold for the type switch, so findFlag is gone. Same idea as #451.

Added TestCliFlagParentFlagsInSubcommand with the three cases above. All three fail on master. go test and go vet pass in providers/cliflagv2.

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