embed: allow symbolic links

[bprosnitz]

Currently, go:embed errors when it is passes a symlinked file, indicating that it isn't supported. This support would be useful.

In the Fuchsia go build system, we often end up with files that are symlinked in a generated directory that serves as inputs to go. While the files can be copied, it would be much easier if the symlinked files could be directly referenced.

[jayconrod]

Sorry, forbidding symbolic links was a design decision from the embed proposal.

It is an error for a pattern to match files outside the current module or that cannot be packaged into a module, like .git/* or symbolic links (or, as noted above, empty directories).

Symbolic links are ignored when modules are archived for distribution. That would mean if symbolic links were allowed, a different set of files would be embedded in a package depending on whether the package was in the main module or in a dependency.

(I'm not familiar with Fuchsia's build system. Is it based on cmd/go or does it invoke cmd/compile directly? cmd/compile treats symbolic links like regular files or directories, but it needs another tool to generate an embedcfg file for it.)