Handle symlinks in go/pkg/tree

[peterebden]

Currently they are silently turned into file nodes of the contents of whatever they point to.

[ola-rozenfeld]

Relevant discussion in #146.

[ola-rozenfeld]

This will need a bit more work before going in:

• fix tests
• add new test cases
• add configuration to Client (Opt) to configure the symlink handling, with current behaviour as the default. Maybe something like:

type DanglingSymlinkResolution int

const (
  DanglingSymlinkResolutionPassThrough DanglingSymlinkResolution = iota
  DanglingSymlinkResolutionIgnore
  DanglingSymlinkResolutionError
)

def SymlinkOpt struct {
  ConvertToFiles bool
  DanglingSymlink DanglingSymlinkResolution
}

We could add more details later, like absolute symlink resolition. This assumes absolute paths will always be converted to files, btw, which I think we should start with (unless BuildGrid supports absolute symlinks?), so the new option will only trigger for relative symlinks.

[peterebden]

As discussed in #146 I don't have capacity to do more here at present, @k-ye is kindly looking into the issue.
Will abandon this, happy for others to reuse parts or ignore as they see fit :)