Improve absolute path handling on Windows

In the presence of forward-slash paths with drive letters.
They can come from environment variable expansion in the proto
configuration.

pkg/filesystem/local_directory_windows.go

@@ -108,10 +108,34 @@ func newLocalDirectory(absPath string, openReparsePoint bool) (DirectoryCloser,
 	return newLocalDirectoryFromHandle(handle)
 }
 
+// Convert forward-slash drive-letter paths to absolute drive-letter paths.
+// This can come from how build directories are configured for bb-worker on Windows.
+// Where a combination of 'git-bash' and environment variable expansion in jsonnet can create paths
+// that the `filepath` does not detect as absolute.
+//
+// Example: /C:/...
+// Should be C:/...
+func CanonicalizeAbsolutePath(path string) string {
+	if len(path) >= 4 && path[0] == '/' && path[2] == ':' && path[3] == '/' {
+		path = path[1:]
+	}
+
+	return path
+}
+
 func NewLocalDirectory(path string) (DirectoryCloser, error) {
-	absPath, err := filepath.Abs(path)
-	if err != nil {
-		return nil, err
+	var absPath string
+	var err error
+
+	path = CanonicalizeAbsolutePath(path)
+
+	if filepath.IsAbs(path) {
+		absPath = filepath.FromSlash(path)
+	} else {
+		absPath, err = filepath.Abs(path)
+		if err != nil {
+			return nil, err
+		}
 	}
 	absPath = "\\??\\" + absPath
 	return newLocalDirectory(absPath, true)

pkg/filesystem/path/resolve.go

@@ -1,6 +1,7 @@
 package path
 
 import (
+	"runtime"
 	"strings"
 
 	"google.golang.org/grpc/codes"
@@ -38,6 +39,12 @@ func (rs *resolverState) push(scopeWalker ScopeWalker, path string) error {
 		path = stripOneOrMoreSlashes(path)
 		absolute = true
 	}
+	if runtime.GOOS == "windows" {
+		// filepath.IsAbs
+		if len(path) > 3 && path[1] == ':' && path[2] == '/' {
+			absolute = true
+		}
+	}
 
 	// Push the path without any leading slashes onto the stack, so
 	// that its components may be processed. Apply  them against the