Fix imported files path lookup on less#lessc linter

Ale saves a temporary file (%t) which does not share the same path as
the original file, breaking import statements with relative URLs.
This fix sends content to `lessc` over stdin and adds
the current file (%s) as one of the included paths, so statements like
`@import '../utils' will correctly resolve based on the current file path.

ale_linters/less/lessc.vim

@@ -4,10 +4,10 @@
 call ale#Set('less_lessc_options', '')
 
 function! ale_linters#less#lessc#GetCommand(buffer) abort
-    return 'lessc'
-    \   . ' --no-color --lint'
+    return 'cat %t | lessc'
+    \   . ' --no-color --lint --include-path=' . expand('%:p:h')
     \   . ' ' . ale#Var(a:buffer, 'less_lessc_options')
-    \   . ' %t'
+    \   . ' -'
 endfunction
 
 function! ale_linters#less#lessc#Handle(buffer, lines) abort