Given the following dependency tree:

  foo
    bar (bottled)
      baz (build-time only)

We skip installing baz because it is a build-time dependency of
something that is bottled. However, during the build of foo, this filter
is not applied because the dependent-dep relationship is not considered
at this stage. If baz wasn't installed prior to this build, fixopt(baz)
will fail.

Further, build-time deps are tightly coupled to the formula they are
specified by, and we shouldn't rely on them coming from dependencies
several levels down.

Fixes Homebrew/homebrew#17697.

This reverts commit 67d06b84d00a17cf43770eedf34e37e50e209741.

Closes Homebrew/homebrew#17683.
Closes Homebrew/homebrew#17685.

This reverts commit 225546a3ee2d3f22e185e47d4db6f6716a56669b.

This allows us to force using the system Ruby on OSX but allow
the development of Homebrew for Linux or Windows to not rely
on OSX paths.

Closes Homebrew/homebrew#17548.

Closes Homebrew/homebrew#17654.

This can cause subtle issues when an exception is marshaled between the
build process and the main Homebrew process, as the marshaled exception
may contain URI objects even when the main Homebrew process has not
loaded the URI library.

Closes Homebrew/homebrew#17642.

Overriding <=> directly is much simpler.

We want to be able to test the raise functionality without
monkey-patching #raise on every object in the system, which is one of
the side effects of loading debrew.rb.

If the debugger's monkey-patched raise was passed an instantiated
exception, the #exception method was called with a potentially nil
argument, causing its instance data to be thrown away. This hides
potentially useful information from the user.

Fix it by allowing instantiated exceptions to be reraised unharmed.

Fixes Homebrew/homebrew#17622.

Closes Homebrew/homebrew#17588.

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

Closes Homebrew/homebrew#17443.

Closes Homebrew/homebrew#17565.

Closes Homebrew/homebrew#11154.

* `CurlDownloadStrategy#_fetch` (and the same methods in its
  subclasses) now fetches the file to a temporary path, and
  `CurlDownloadStrategy#fetch` moves it to the correct location.
* `Homebrew#cleanup` cleans the temporary files `CurlDownloadStrategy`
  creates if they're left in the cache.

Closes Homebrew/homebrew#13953.

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