Set makeprg if pip-compile is available

also update tests and make Requirements_matched_filename more specific.

after/ftplugin/requirements.vim

@@ -0,0 +1,11 @@
+if executable('pip-compile')
+    let s:filename = expand("%:p")
+    if fnamemodify(s:filename, ":t") ==# 'requirements.in'
+        " this is the default filename for pip-compile
+        setlocal makeprg=pip-compile
+    elseif fnamemodify(s:filename, ":e") ==# 'in'
+        \ && Requirements_matched_filename(s:filename)
+        setlocal makeprg=pip-compile\ %
+    endif
+endif
+" vim: et sw=4 ts=4 sts=4: