Fix HandleLSPDiagnostics buffer match logic.

To find the buffer corresponding to URIs reported by LSP the
HandleLSPDiagnostics() method uses the built-in bufnr() function. From
the documentation we learn that the first parameter of bufnr() is
an expression, not a path.

EclipseLSP will report project wide errors (e.g. gradle errors) that are
not related to any actual source file with an URI that corresponds to the
project root folder, e.g:

   file:///home/username/Projects/gradle-simple

This URI will match any open buffer of files within the project root
hiearchy, thus project-wide errors appear as part of every file within
the project, e.g:

   file:///home/username/Projects/gradle-simple/src/main/java/Hello.java

To fix this, this MR adds '^' to the beginning and '$' at the end of the
URI path to force an exact match. This is how is recommended in vim
help (see :h bufname).

autoload/ale/lsp_linter.vim

@@ -31,7 +31,7 @@ endfunction
 function! s:HandleLSPDiagnostics(conn_id, response) abort
     let l:linter_name = s:lsp_linter_map[a:conn_id]
     let l:filename = ale#path#FromURI(a:response.params.uri)
-    let l:buffer = bufnr(l:filename)
+    let l:buffer = bufnr('^' . l:filename . '$')
     let l:info = get(g:ale_buffer_info, l:buffer, {})
 
     if empty(l:info)