Switch flag definitions to be on pflag instead of flag in `package go/cmd/vtexplain`

[ajm188]

Part of #10697.

Current flags:

$ git grep -E "\bflag\.[A-Z]" -- go/cmd/vtexplain/*.go
        sqlFlag            = flag.String("sql", "", "A list of semicolon-delimited SQL commands to analyze")
        sqlFileFlag        = flag.String("sql-file", "", "Identifies the file that contains the SQL commands to analyze")
        schemaFlag         = flag.String("schema", "", "The SQL table schema")
        schemaFileFlag     = flag.String("schema-file", "", "Identifies the file that contains the SQL table schema")
        vschemaFlag        = flag.String("vschema", "", "Identifies the VTGate routing schema")
        vschemaFileFlag    = flag.String("vschema-file", "", "Identifies the VTGate routing schema file")
        ksShardMapFlag     = flag.String("ks-shard-map", "", "JSON map of keyspace name -> shard name -> ShardReference object. The inner map is the same as the output of FindAllShardsInKeyspace")
        ksShardMapFileFlag = flag.String("ks-shard-map-file", "", "File containing json blob of keyspace name -> shard name -> ShardReference object")
        numShards          = flag.Int("shards", 2, "Number of shards per keyspace. Passing --ks-shard-map/--ks-shard-map-file causes this flag to be ignored.")
        executionMode      = flag.String("execution-mode", "multi", "The execution mode to simulate -- must be set to multi, legacy-autocommit, or twopc")
        replicationMode    = flag.String("replication-mode", "ROW", "The replication mode to simulate -- must be set to either ROW or STATEMENT")
        normalize          = flag.Bool("normalize", false, "Whether to enable vtgate normalization")
        outputMode         = flag.String("output-mode", "text", "Output in human-friendly text or json")
        dbName             = flag.String("dbname", "", "Optional database target to override normal routing")
        plannerVersionStr  = flag.String("planner-version", "", "Sets the query planner version to use when generating the explain output. Valid values are V3 and Gen4")
        badPlannerVersion  = flag.String("planner_version", "", "Deprecated flag. Use planner-version instead")
        flag.CommandLine.SetOutput(logutil.NewLoggerWriter(logger))
        _flag.SetUsage(flag.CommandLine, _flag.UsageOptions{
                FlagFilter: func(f *flag.Flag) bool {