Write all logging (INFO, WARN, ERROR) to stderr

The old setup tried to log >= warning to stderr, the rest to stdout.

However, that logic was flawed, so warnings ended up in stdout, which makes `hugo list all` etc. hard to reason about from scripts.

This commit fixes this by making all logging (info, warn, error) log to stderr and let stdout be reserved for program output.

Fixes #13074

testscripts/commands/deprecate.txt

@@ -1,13 +1,13 @@
 
 # Test deprecation logging.
 hugo -e info --logLevel info
-stdout 'INFO  deprecated: item was deprecated in Hugo'
+stderr 'INFO  deprecated: item was deprecated in Hugo'
 
 hugo -e warn --logLevel warn
-stdout 'WARN  deprecated: item was deprecated in Hugo'
+stderr 'WARN  deprecated: item was deprecated in Hugo'
 
 ! hugo -e error --logLevel warn
-stdout 'ERROR deprecated: item was deprecated in Hugo'
+stderr 'ERROR deprecated: item was deprecated in Hugo'
 
 -- hugo.toml --
 baseURL = "https://example.com/"

testscripts/commands/hugo__configdir.txt

@@ -3,4 +3,5 @@ hugo
 ! stderr .
 
 -- config/_default/hugo.toml --
-baseURL = "https://example.com/"
+baseURL = "https://example.com/"
+disableKinds = ["RSS", "page", "sitemap", "robotsTXT", "404", "taxonomy", "term", "home"]

testscripts/commands/hugo__path-warnings.txt

@@ -1,6 +1,6 @@
 hugo --printPathWarnings
 
-stdout 'Duplicate'
+stderr 'Duplicate'
 
 -- hugo.toml --
 -- assets/css/styles.css --

testscripts/commands/hugo_printpathwarnings.txt

@@ -1,6 +1,6 @@
 hugo  --printPathWarnings
 
-stdout 'Duplicate target paths: .index.html \(2\)'
+stderr 'Duplicate target paths: .index.html \(2\)'
 
 -- hugo.toml --
 disableKinds = ["taxonomy", "term", "RSS", "sitemap", "robotsTXT", "404", "section"]

testscripts/commands/hugo_printunusedtemplates.txt

@@ -1,6 +1,6 @@
 hugo  --printUnusedTemplates
 
-stdout 'Template _default/list.html is unused'
+stderr 'Template _default/list.html is unused'
 
 -- hugo.toml --
 disableKinds = ["taxonomy", "term", "RSS", "sitemap", "robotsTXT", "404", "section", "page"]

testscripts/commands/warnf_stderr.txt

@@ -0,0 +1,13 @@
+# Issue #13074
+
+hugo
+stderr 'warning'
+! stdout 'warning'
+
+-- hugo.toml --
+baseURL = "http://example.org/"
+disableKinds = ["RSS", "page", "sitemap", "robotsTXT", "404", "taxonomy", "term"]
+-- layouts/index.html --
+Home
+{{ warnf "This is a warning" }}
+