compiler cleanup

Now using the same pattern as Go's http package.

- the `compiler.Compiler` struct can be used directly (and tests to do
  to avoid messing with the global version)
- `compiler.DefaultCompiler` contains a public default Compiler instance
- `compiler` exposes proxy functions (e.g. Compile) back to the
  DefaultCompiler

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

dagger/compiler/utils.go

@@ -0,0 +1,22 @@
+package compiler
+
+import (
+	"cuelang.org/go/cue"
+)
+
+func cueStringsToCuePath(parts ...string) cue.Path {
+	selectors := make([]cue.Selector, 0, len(parts))
+	for _, part := range parts {
+		selectors = append(selectors, cue.Str(part))
+	}
+	return cue.MakePath(selectors...)
+}
+
+func cuePathToStrings(p cue.Path) []string {
+	selectors := p.Selectors()
+	out := make([]string, len(selectors))
+	for i, sel := range selectors {
+		out[i] = sel.String()
+	}
+	return out
+}