uninstall: remove nil requireds/dependents

development_tools: don't create Versions from `nil`

uninstall: skip some flaky tests for now.

This cause issues when e.g. using `debrew.rb` on a failing `system`
command in a formula.

These shouldn’t get here in the first place so this is a bit of a hack
pending a better fix.

These will be fixed properly later but this will avoid causing
unnecessary CI failures.

audit: fix "version should not decrease" (again).

- output the max version and current version to make the message more
  obvious.
- Don't compare the development spec version to the formula version and
  then complain the formula version is lower (it should always be!)

Warn developers when uninstalling a dependency

Add "null version" class, and return compiler versions/build versions as Version objects

Update RuboCop to 0.45.0.

Fix detection of self-extracting `.exe` files.

Works around Rubycop not liking method names that start with `is_`
by changing convention from singular to plural.

I think it's better that way anyway.

Suggested in #1084.

Made the existing warning output entirely to STDERR, because
previously the first line went to STDERR and subsequent ones went
to STDOUT.

Don't warn about unused `brew install` arguments.

This is a step closer to better argument handling but for now just
fixes the issue in #1217 where it starts complaining about options like
`--build-from-source` being used.

formula_versions: fix erroneous :stable in version_attributes_map

cc: Add -frounding-math to list of ignored flags

keg: don't rely on #to_formula

It doesn't always work.
For example, a keg could have been installed with a formula from a
URL, which Homebrew now does not know how to access.

Fixes #1496.

audit: fix "version should not decrease" check.

Fix the "version should not decrease" check so it correctly handles
`version_scheme`s.

Fixes #1489.

Update macOS Xcode tool checks do use Version class for comparisons

Invalid build option warnings - supersedes #1088

Filter formula build options based on those that exist.

diagnostic: skip xcode-select check with no Xcode.