System duplicates can now provide more information about why
they are included in Homebrew.

Fixes Homebrew/homebrew#4683

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

If version cannot be determined, then instruct the user to add one.

Because we should always be able to help. And I've checked, it works fine.

Be more useful by being more concise. I referenced dozens of other mature commands’ usage to figure out what to do here.

Also separated out the help into its own command for consistency.

Because since 0.8 we do the opposite, ie. download by default.

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

This provides a uniform way for formulae to indicate a Fortran compiler
is required.

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

Prefer a shallow clone for builds from git repos, as we don't need all
the historical stuff.

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

When a cellar exists for an unknown formula, no longer print a warning.
External tools using Brew to manage diy-installs will create a large
number of non-formula cellars.

The refactor branch doesn't catch the UEDE and print the error message
containing information about which dependency and how to install
it. This adds back that logic.

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