Slang verilog Linter: set default searchpath and output filename (#4999)

* Sets module search path to file directory for slang/verilog linter

Similar to iverilog behaviour.
See 699c0dbe80

Reasonable default behaviour, can be changed by adding '-y%s:h' to
`b:ale_verilog_slang_options` buffer variable.

* Adds filename to slang/verilog linter output

slang can parse other files based on the modules instances names find in
the current file and returns warning/error messages related to those
files that have the same pattern.

Adding the file name to the outputs avoid polluting the active buffer
with those messages.

* Absolute paths in slang/verilog linter messages

Otherwise temp files are not correctly detected

ale_linters/verilog/slang.vim

@@ -9,7 +9,9 @@ endif
 " --lint-only
 function! ale_linters#verilog#slang#GetCommand(buffer) abort
     return 'slang -Weverything '
+    \   . '--diag-abs-paths '
     \   . '-I%s:h '
+    \   . '-y%s:h '
     \   . ale#Var(a:buffer, 'verilog_slang_options') .' '
     \   . '%t'
 endfunction
@@ -28,6 +30,7 @@ function! ale_linters#verilog#slang#Handle(buffer, lines) abort
 
     for l:match in ale#util#GetMatches(a:lines, l:pattern)
         let l:item = {
+        \   'filename': l:match[1],
         \   'lnum': str2nr(l:match[2]),
         \   'type': (l:match[4] is# 'error') ? 'E' : 'W',
         \   'text': s:RemoveUnicodeQuotes(l:match[5]),