Create a struct with all of Hugo's config options

Primary motivation is documentation, but it will also hopefully simplify the code.

Also,

* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.

Closes #10896
Closes #10620

common/urls/baseURL.go

@@ -0,0 +1,110 @@
+// Copyright 2023 The Hugo Authors. All rights reserved.
+//
+// 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 urls
+
+import (
+	"fmt"
+	"net/url"
+	"strconv"
+	"strings"
+)
+
+// A BaseURL in Hugo is normally on the form scheme://path, but the
+// form scheme: is also valid (mailto:hugo@rules.com).
+type BaseURL struct {
+	url         *url.URL
+	WithPath    string
+	WithoutPath string
+	BasePath    string
+}
+
+func (b BaseURL) String() string {
+	return b.WithPath
+}
+
+func (b BaseURL) Path() string {
+	return b.url.Path
+}
+
+func (b BaseURL) Port() int {
+	p, _ := strconv.Atoi(b.url.Port())
+	return p
+}
+
+// HostURL returns the URL to the host root without any path elements.
+func (b BaseURL) HostURL() string {
+	return strings.TrimSuffix(b.String(), b.Path())
+}
+
+// WithProtocol returns the BaseURL prefixed with the given protocol.
+// The Protocol is normally of the form "scheme://", i.e. "webcal://".
+func (b BaseURL) WithProtocol(protocol string) (BaseURL, error) {
+	u := b.URL()
+
+	scheme := protocol
+	isFullProtocol := strings.HasSuffix(scheme, "://")
+	isOpaqueProtocol := strings.HasSuffix(scheme, ":")
+
+	if isFullProtocol {
+		scheme = strings.TrimSuffix(scheme, "://")
+	} else if isOpaqueProtocol {
+		scheme = strings.TrimSuffix(scheme, ":")
+	}
+
+	u.Scheme = scheme
+
+	if isFullProtocol && u.Opaque != "" {
+		u.Opaque = "//" + u.Opaque
+	} else if isOpaqueProtocol && u.Opaque == "" {
+		return BaseURL{}, fmt.Errorf("cannot determine BaseURL for protocol %q", protocol)
+	}
+
+	return newBaseURLFromURL(u)
+}
+
+func (b BaseURL) WithPort(port int) (BaseURL, error) {
+	u := b.URL()
+	u.Host = u.Hostname() + ":" + strconv.Itoa(port)
+	return newBaseURLFromURL(u)
+}
+
+// URL returns a copy of the internal URL.
+// The copy can be safely used and modified.
+func (b BaseURL) URL() *url.URL {
+	c := *b.url
+	return &c
+}
+
+func NewBaseURLFromString(b string) (BaseURL, error) {
+	u, err := url.Parse(b)
+	if err != nil {
+		return BaseURL{}, err
+	}
+	return newBaseURLFromURL(u)
+
+}
+
+func newBaseURLFromURL(u *url.URL) (BaseURL, error) {
+	baseURL := BaseURL{url: u, WithPath: u.String()}
+	var baseURLNoPath = baseURL.URL()
+	baseURLNoPath.Path = ""
+	baseURL.WithoutPath = baseURLNoPath.String()
+
+	basePath := u.Path
+	if basePath != "" && basePath != "/" {
+		baseURL.BasePath = basePath
+	}
+
+	return baseURL, nil
+}