Optimize downloadable sources

Description

This optimizes some hot path codes for all downloadable sources.

While targeted to a certain type of sources, most used sources (tar, deb, etc) are derived from this one.

We should therefore see a slight improvement on any workload.

Changes proposed in this merge request:

• cythonize url_directory_name
• Precompute and cache some often accessed values

Benchmarks:

commit	action	python_version	max_memory	time
bschubert/optimize-downloadable-sources - d26b7ed0	show	py37	1642	58.74
master - fbb8eea8	show	py37	1595	61.21

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