raco pkg: allow read operations on read-only package data

In particular, `raco pkg show' should work when the user does not
have write access to the installation directory or installation-wide
package database.

Merge to v5.3.2
bug report GUI: guard against `raco pkg show' errors

Various things can go wrong with `raco pkg show', including
permission errors or lock conflicts.

Merge to v5.3.2