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

test/handler/test_slang_handler.vader

@@ -8,12 +8,14 @@ Execute(The slang handler should parse lines correctly):
   AssertEqual
   \ [
   \    {
+  \      'filename' : 'foo.sv',
   \      'lnum': 11,
   \      'col': 1,
   \      'type': 'W',
   \      'text': 'extra '';'' has no effect [-Wempty-member]',
   \    },
   \    {
+  \      'filename' : 'bar.sv',
   \      'lnum': 24,
   \      'col': 12,
   \      'type': 'E',
@@ -22,5 +24,5 @@ Execute(The slang handler should parse lines correctly):
   \ ],
   \ ale_linters#verilog#slang#Handle(bufnr(''), [
   \ 'foo.sv:11:1: warning: extra '';'' has no effect [-Wempty-member]',
-  \ 'foo.sv:24:12: error: cannot mix continuous and procedural assignments to variable ''data_o''',
+  \ 'bar.sv:24:12: error: cannot mix continuous and procedural assignments to variable ''data_o''',
   \ ])

test/linter/test_slang.vader

@@ -5,10 +5,10 @@ After:
   call ale#assert#TearDownLinterTest()
 
 Execute(The default slang command should be correct):
-  AssertLinter 'slang', 'slang -Weverything -I%s:h  %t'
+  AssertLinter 'slang', 'slang -Weverything --diag-abs-paths -I%s:h -y%s:h  %t'
 
 Execute(slang options should be configurable):
   " Additional args for the linter
   let g:ale_verilog_slang_options = '--define-macro DWIDTH=12'
 
-  AssertLinter 'slang', 'slang -Weverything -I%s:h --define-macro DWIDTH=12 %t'
+  AssertLinter 'slang', 'slang -Weverything --diag-abs-paths -I%s:h -y%s:h --define-macro DWIDTH=12 %t'