Closes Homebrew/homebrew#32472.

As this is something we support and update.

You can use any number of "/" operators, just as you can "+".

And don't uninstall them in-between. This should fix the issues where
if e.g. OpenSSL changes in a pull request then it's only the dependents
after it alphabetically would be built against the new version.

This fixes broken output like this:

  Error: No such file or directory - non_existent_file
  ee/src/tree-1.7.0.tgz
  Already downloaded: /Library/Caches/Homebrew/tree-1.7.0.tgz
  ==> Verifying tree-1.7.0.tgz checksum
  tar xf /Library/Caches/Homebrew/tree-1.7.0.tgz

Closes Homebrew/homebrew#32145.

Closes Homebrew/homebrew#30540.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

If both a formula and its dependencies are changed in a single pull
request it'll fail because the bottle block may no longer be correct.

Handle this case by ignoring bottle pour failures and fetching the
source packages instead of bottles.

This fix means that brew-test-bot can bottle formulae that require GCC
again.