import: pypi: Improve parsing of requirement specifications.

import: pypi: Improve parsing of requirement specifications.

The previous solution was fragile and could leave unwanted characters in a
requirement name, such as '[' or ']'.

Partially fixes <https://bugs.gnu.org/33047>.

* guix/import/pypi.scm (use-modules): Export SPECIFICATION->REQUIREMENT-NAME
(%requirement-name-regexp): New variable.
(clean-requirement): Rename to...
(specification->requirement-name): this, which now uses
%requirement-name-regexp to select the requirement name from the requirement
specification.
(parse-requires.txt): Adapt.