`brew install ./foo` treats `./foo` like `./foo.rb`. This requires
a confusing special case in the name resolution logic and doesn't make
a whole lot of sense from a UX perspective.

Drop support for this. The argument should be an absolute path,
a relative path, or a formula name, rather than a hybrid.

cf. https://github.com/Homebrew/homebrew/issues/23430#issuecomment-26776962

Now we can avoid computing the path twice in the common case.

Much of the name resolution done in Formula.canonical_name is repeated
Formulary.factory. Here we eliminate the repeated work by duplicating
the code from canonical_name. Later we will refactor it so that both
methods can share the bulk of the logic.

Fixes Homebrew/homebrew#28188.

Symlinks in opt and LinkedKegs point directly at a keg in the cellar, so
only resolving one symlink should suffice, and make it clear what path
we are actually interested in.

Closes Homebrew/homebrew#21223.

Fixes Homebrew/homebrew#28144.

Closes Homebrew/homebrew#28173.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

This hack is starting to outlive its usefulness, and produces false
positives.

Fixes Homebrew/homebrew#28147.

This is as Python bindings are not build in a way that can have
system or Homebrew Python used interchangeably.

Allows disabling bottles from requirements.

As pointed out by @mikemcquaid in http://git.io/Q5b9wg



Closes Homebrew/homebrew#28106.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

This reverts commit 557ef8df05a8ea28ed903f08b2009ebc0e218f5a.

Closes Homebrew/homebrew#28095.

Closes Homebrew/homebrew#28094.

MacOS.version#<=> is called many, many times during formula loading with
the same half dozen or so arguments. A typical call to this method
involves:

 * a hash lookup to convert a symbol argument to a string
 * creation of a throw-away Version object wrapping the argument
 * the actual version comparison, which is not cheap

This makes it a prime candidate to be memoized.

The search mechanism in MacOS::Xcode is very slow. It requires shelling
out at least twice, and possibly a third time (in the CLT-only case).

Calling provides_cvs? activates this in order to determine the Xcode
version. But if we know that there isn't an Xcode available for the
current OS that meets the criteria, we can avoid this check entirely.