Detect shivammathur/homebrew-php conflicts

[nicoverbruggen]

#54 is an issue right now and could be a problem even once Homebrew core is updated and PHP aliases match up if the user hasn't updated Homebrew in a while. Because of this, PHP Monitor is "broken" but a workaround might be possible.

(This conflict detection might be necessary if shivammathur/php is found in the output of brew tap?)

It is possible to determine if aliases are conflicting by running:

brew info shivammathur/php/php --json

If the alias there does not match the one in:

brew info php --json

We should show a warning and, if possible, perhaps alter the behaviour of the linker to ensure no duplicates are linked. Perhaps it's possible to unlink the conflict after linking php?

[nicoverbruggen]

This has been added now, along with a few other changes that will make it easier to check for PHP installation issues in the future.