lint: Improve check for synopses starting with package name.

* guix/scripts/lint.scm (package-name-regexp): New procedure.
  (check-synopsis-style)[check-start-with-package-name]: Use it instead
  of 'string-prefix-ci?'.
* tests/lint.scm ("synopsis: start with package name prefix"): New test.

guix/scripts/lint.scm

@@ -131,6 +131,12 @@ (define (check-inputs-should-be-native package)
                       "pkg-config should probably be a native input"
                       'inputs))))))
 
+(define (package-name-regexp package)
+  "Return a regexp that matches PACKAGE's name as a word at the beginning of a
+line."
+  (make-regexp (string-append "^" (regexp-quote (package-name package))
+                              "\\>")
+               regexp/icase))
 
 (define (check-synopsis-style package)
   ;; Emit a warning if stylistic issues are found in the synopsis of PACKAGE.
@@ -168,7 +174,7 @@ (define (check-proper-start synopsis)
                     'synopsis)))
 
   (define (check-start-with-package-name synopsis)
-    (when (string-prefix-ci? (package-name package) synopsis)
+    (when (regexp-exec (package-name-regexp package) synopsis)
       (emit-warning package
                     "synopsis should not start with the package name"
                     'synopsis)))

tests/lint.scm

@@ -194,6 +194,14 @@ (define (call-with-warnings thunk)
                           (check-synopsis-style pkg))))
                     "synopsis should not start with the package name")))
 
+(test-assert "synopsis: start with package name prefix"
+  (string-null?
+   (call-with-warnings
+    (lambda ()
+      (let ((pkg (dummy-package "arb"
+                   (synopsis "Arbitrary precision"))))
+        (check-synopsis-style pkg))))))
+
 (test-assert "inputs: pkg-config is probably a native input"
   (->bool
    (string-contains