core: Implement FastAbs to avoid repeated os.Getwd calls (#6687)

* core: Implement FastAbs to avoid repeated os.Getwd calls

* Lint

* Rename files

filepath_windows.go

@@ -0,0 +1,25 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package caddy
+
+import (
+	"path/filepath"
+)
+
+// FastAbs can't be optimized on Windows because the
+// syscall.FullPath function takes an input.
+func FastAbs(path string) (string, error) {
+	return filepath.Abs(path)
+}