Re-work ARGV filtering to properly handle --HEAD

Previously, stripping arguments like `--HEAD` for dependencies failed because
that flag affects the installation prefix encoded into formula objects. The
previous implementation of `ARGV` filtering tried to contain all changes to a
single method call before the `FormulaInstaller` forks. This update spreads
things out a bit:

  - The Homebrew `ARGV` extension adds a new method, `filter_for_dependencies`
    which strips flags like `--HEAD`, yields to a block, then restores the
    original contents of ARGV.

  - The `explicitly_requested?` test, which returns true or false depending on
    if a formula object is a member of `ARGV.formulae`, is now a method of
    `Formula` objects.

  - `FormulaInstaller` objects now execute the installation of dependencies
    inside an `ARGV.filter_for_dependencies` block if the dependency was
    `explicitly_requested?`.

Fixes #8668.
Closes #7724.

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