Fixes Homebrew/homebrew#19013.
Fixes Homebrew/homebrew#19862.
Fixes Homebrew/homebrew#19921.

This is now available from PyPI, and the github tarball does not contain
all of the necessary dependencies, so remove it and recommend "pip
install gsutil".

Closes Homebrew/homebrew#19874.
Closes Homebrew/homebrew#19895.

Rationale: "prune" is slow, does not need to be run as often as
"cleanup", and is potentially destructive to things like empty
directories that Homebrew did not create. Thus we should let users run
`brew prune` only when they wish to, while still being able to use `brew
cleanup`.

Closes Homebrew/homebrew#19863.

This information is only used by the downloader, no need to expose it.

This information is already exposed as "cached_location", which is
common to all download strategies.

Fixes Homebrew/homebrew#19873.

This allows us to remove some type checks that were protecting against
potential nils.

It doesn't really make logical sense that this method returns both the
fetched path (or sometimes nil!) and the downloader, so just return the
path (again, or nil!) and callers that want the downloader can ask for
it separately.

Closes Homebrew/homebrew#19826.

If f.installed? is true, f.rack.directory? will always be true as well.

Closes Homebrew/homebrew#19828.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

The version isn't redundant if a subclass of Version is specified.

It is possible for the object returned by Formula#version to be a
subclass of Version with special behavior, so we want to use that for
the comparison.

If an incomplete download exists, and the HTTP server doesn't support
byte ranges, delete the incomplete download and retry once.

Tested with freeling, whose server does not support byte ranges.

Closes Homebrew/homebrew#19757.

The CFLAGS were previously not generic enough.

References Homebrew/homebrew#18944.
References Homebrew/homebrew#19179.

Fixes Homebrew/homebrew#17550.
Closes Homebrew/homebrew#19668.