Returns True if the given package (given in pip's package syntax or a
        tuple of ('name', 'ver')) is installed in the virtual environment.

def is_installed(self, package):
        """Returns True if the given package (given in pip's package syntax or a
        tuple of ('name', 'ver')) is installed in the virtual environment."""
        if isinstance(package, tuple):
            package = '=='.join(package)
        if package.endswith('.git'):
            pkg_name = os.path.split(package)[1][:-4]
            return pkg_name in self.installed_package_names or \
                    pkg_name.replace('_', '-') in self.installed_package_names
        pkg_tuple = split_package_name(package)
        if pkg_tuple[1] is not None:
            return pkg_tuple in self.installed_packages
        else:
            return pkg_tuple[0].lower() in self.installed_package_names