Closes Homebrew/homebrew#20676.

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

Fixes Homebrew/homebrew#20673.

Fixes Homebrew/homebrew#20640.

Check for HOMEBREW_PREFIX/share/python and python3
in PATH and tell about the new location and that
files from there can be removed. Also instructions
on how to upgrade python packages are given.

Some prefer to install with `--user` into their
home. For example to avoid `sudo` with system's
python. Now Homebrew can use those modules to
satisfy a python dependency on a certain module.

For example python and python3 both provide a
`Python.framework` and OS X Frameworks are made to
deal with this by putting them into `Versions` and
linking the default one as `Current`. However, brew
did not respect this and cleaned out stuff making
neither of the two versions work because `Current`
is replaced by an empty directory.

This commit fixes that. However, still one of the
two `Python.frameworks` has to remove it's
`Current` and `Headers` links, because brew cannot
decide alone which one is the "default" and allowed
to set the `Current` link.

A similar situation might apply to Qt 4 vs. 5..

* Fixes Homebrew/homebrew#20572 by tweaking the logic that decides
  which python is used by the `python` object
  inside a formula. There was a bug when on 10.6
  there is no Python 2.7 but a :recommended
  Python was still treated as being available.

* Use the user's PATH when looking for an external
  Python. Until now only brewed or OS X system's
  python have been found by `depends_on :python`.

  But now we support any Python in PATH (e.g.
  pyenv's python).

* Further, instead of handling python modules
  and import tests in LanguageModuleDependency,
  these are now handled by:

        depends_on :python => 'numpy'  # for example

  The old style

        depends_on 'numpy' => :python

  is still supported and is only an alias
  for the newer style (only for :python, the
  other languages are not altered by this commit).

  The reasoning is that if a formula requires
  a python module, it basically also needs
  python itself - and further that specific
  version of python has to provide the module.
  So the `PythonInstalled` is the natural place
  to check for the availability of a python
  module.

  Using a python module and other tags like
  :optional or :recommended is done like so:

        depends_on :python => [:optional, 'numpy']

  Specifying another PyPi (Python Package index)
  name than the module import name is seldom used
  but supported, too:

         depends_on :python => ['enchant'=>'pyenchant']

  A last note: For clarity, you can define
  multiple depends_on statements with different
  modules to be importable.`

CLT-only builds are still broken and need to be fixed at some point.

Closes Homebrew/homebrew#20524.

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

Closes Homebrew/homebrew#19998.

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

This command loads all formulae and dumps an HTML profile report to
stdout. So e.g. `brew profile >/tmp/report.html`

Exposing this as "to_a" was a mistake, versions are not arrays and it
causes incorrect behavior when splatted or using Kernel#Array(). Use the
more correct name "tokens" instead.

Closes Homebrew/homebrew#20507.

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