language/go: generate embedsrcs attribute from //go:embed directive

[jayconrod]

This is the Gazelle half of bazel-contrib/rules_go#2775.

Gazelle should recognize embed directives like this (added in Go 1.16):

import "embed"

//go:embed static/*
var staticFiles embed.FS

Gazelle should match embed patterns against files in the current directory, then list those files in an embedsrcs attribute, which would make them available for embedding.

go_library(
    name = "server",
    importpath = "example.com/server",
    srcs = ["server.go"],
    embedsrcs = [
        "a.html",
        "b.html",
    ],
)