Simplify the code for most linters and tests with closures

ale_linters/go/gometalinter.vim

@@ -5,12 +5,7 @@ call ale#Set('go_gometalinter_options', '')
 call ale#Set('go_gometalinter_executable', 'gometalinter')
 call ale#Set('go_gometalinter_lint_package', 0)
 
-function! ale_linters#go#gometalinter#GetExecutable(buffer) abort
-    return ale#Var(a:buffer, 'go_gometalinter_executable')
-endfunction
-
 function! ale_linters#go#gometalinter#GetCommand(buffer) abort
-    let l:executable = ale_linters#go#gometalinter#GetExecutable(a:buffer)
     let l:filename = expand('#' . a:buffer . ':t')
     let l:options = ale#Var(a:buffer, 'go_gometalinter_options')
     let l:lint_package = ale#Var(a:buffer, 'go_gometalinter_lint_package')
@@ -19,12 +14,12 @@ function! ale_linters#go#gometalinter#GetCommand(buffer) abort
     " be calculated to absolute paths in the Handler
     if l:lint_package
         return ale#path#BufferCdString(a:buffer)
-        \   . ale#Escape(l:executable)
+        \   . '%e'
         \   . (!empty(l:options) ? ' ' . l:options : '') . ' .'
     endif
 
     return ale#path#BufferCdString(a:buffer)
-    \   . ale#Escape(l:executable)
+    \   . '%e'
     \   . ' --include=' . ale#Escape(ale#util#EscapePCRE(l:filename))
     \   . (!empty(l:options) ? ' ' . l:options : '') . ' .'
 endfunction
@@ -55,7 +50,7 @@ endfunction
 
 call ale#linter#Define('go', {
 \   'name': 'gometalinter',
-\   'executable_callback': 'ale_linters#go#gometalinter#GetExecutable',
+\   'executable_callback': ale#VarFunc('go_gometalinter_executable'),
 \   'command_callback': 'ale_linters#go#gometalinter#GetCommand',
 \   'callback': 'ale_linters#go#gometalinter#Handler',
 \   'lint_file': 1,