go: support //go:embed directives with embedsrcs attribute (#2806)

go_library, go_binary, and go_test now support the //go:embed
directive in Go 1.16. When building a package containing a //go:embed
directive, they'll match patterns against embedable files to generate
an embedcfg file to pass to the compiler.

Embeddable files must be listed in the new embedsrcs attribute.

The GoSource provider now has an embedsrcs field to support this.

Fixes #2775