formula: fix specified_path for aliases without core tap

[Bo98]

This previously assumed a non-nil alias_path would always exist where the unresolved path exists. This is not always true with untapped core taps - e.g. if we download the Ruby source or use it from a bottle (post-install) then the we have the original file but not the aliased symlink.

Fixes some errors installing by alias, e.g.

Error: An exception occurred within a child process:
  FormulaUnavailableError: No available formula with the name "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Aliases/crypto++".

Technically speaking this means methods like alias_name etc will not be set in postinstall etc, but I don't think any formula uses that information and would be a bigger refactor to fix.

[MikeMcQuaid]

Thanks again @Bo98!