fix: performance regression when parsing links from legacy repositories (#6442)

Resolves: #6436

Measurements of `poetry lock` with warm cache with example
pyproject.toml from #6436:

|test case|time in s|peak memory usage in MB|
|---|---|---|
|legacy repository (before)|422|113|
|legacy repository (after)|3|118|
|pypi repository|1|92|

`backports.cached-property` is used in order to support
cached_property on Python 3.7. 

Co-authored-by: Jarrod Moore <jmo@jmo.name>
Co-authored-by: Bjorn Neergaard <bjorn@neersighted.com>