Remove Cask developer scripts.

Add permit_weak_imports directive

Formula-Cookbook: document head branch default.

`--HEAD` defaults to `:branch => "master"` so document that.

Closes #722.

diagnostic: ignore duplicated `brew-cask` commands

Bottles: bottle revision is now called rebuild.

download_strategy: check if commit nil

These are intentionally duplicated for now until the Cask migration is
complete.

Move Cask core code to Homebrew

Forgot this in #731.

Rename bottle's revision to rebuild.

xcode: update expected clang for macOS 10.12

Closes #746.

Signed-off-by: ilovezfs <ilovezfs@icloud.com>

version: fix parsing of URLs ending with version

Add Formula Cookbook entry and audit check for version_scheme

Reference the revision value so it's more obvious that this does not
refer to e.g. a SCM revision.

Reuse the existing `revision` method that was made more generic.

This will allow it to be used for checking other attributes too.

This allows querying multiple attributes in the same way as
`revision_map` did but without duplicating code or repeatedly traversing
history.

https://github.com/lihaoyi/Ammonite/releases/download/0.7.4/0.7.4

was parsing as "0.7" not "0.7.4" since ".4" was being treated as a
legitimate file extension. At minimum, let's insist that a valid file
extension include at least one letter to avoid lopping off part of the
correct version string.

Implement epochs

Brew-Test-Bot.md: fix broken link

Issue Homebrew/homebrew-core#3727 suggested we set -no_weak_imports for
new versions of Xcode to ensure that e.g. building on 10.11 against the
10.12 SDK doesn't result in a situation where autotools thinks symbols
exist that don't actually exist on the current platform.

Further discussion in golang/go#16770 revealed that some packages
require weak imports to build normally.