Avoid error on analyzing non-racket files

When `resyntax-analyze` is called directly on a non-`#lang racket` file, it currently results in an error during macro expansion being raised and logged. This causes issues when the langserver tries to analyze the current file with Resyntax, see jeapostrophe/racket-langserver#153. This PR moves Resyntax's logic for handling such files out of the file group resolution code and into the core of `resyntax-analyze`.

Author: jackfirth

main.rkt

@@ -142,11 +142,15 @@
     (git-commit! message)))
 
 
-(define (resyntax-analyze source
+(define/guard (resyntax-analyze source
                           #:suite [suite default-recommendations]
                           #:lines [lines (range-set (unbounded-range #:comparator natural<=>))])
   (define comments (with-input-from-source source read-comment-locations))
   (define full-source (source->string source))
+  (guard (string-prefix? full-source "#lang racket") #:else
+    (log-resyntax-warning "skipping ~a because it does not start with #lang racket"
+                          (or (source-path source) "string source"))
+    (refactoring-result-set #:base-source source #:results '()))
   (log-resyntax-info "analyzing ~a" (or (source-path source) "string source"))
   (for ([comment (in-range-set comments)])
     (log-resyntax-debug "parsed comment: ~a: ~v" comment (substring-by-range full-source comment)))

private/file-group.rkt

@@ -116,9 +116,7 @@
 
 (define/guard (rkt-file? portion)
   (define path (file-portion-path portion))
-  (guard (path-has-extension? path #".rkt") #:else #false)
-  (define content (file->string path))
-  (string-prefix? content "#lang racket"))
+  (guard (path-has-extension? path #".rkt") #:else #false)))
 
 
 ;; GitHub allows pull request reviews to include comments only on modified lines, plus the 3 lines