Avoid a 'Sys_error' with 'opam pin'

When pinning to a local directory (or a git), if the source directory
contains files in 'opam/files', opam may raise

  Sys_error(.../opam/files/ocsigenserver.install: No such file or directory)'
Merge pull request #1421 from hnrgrgr/fix_not_found_pin_files

Avoid a 'Sys_error' with 'opam pin'