Metadata caching

[gaborcsardi]

• there is a five minutes cache
• it performs a HEAD request first
• downloads are performed in parallel
• it always downloads PACKAGES.gz files for now
• converts PACKAGES.gz to RDS
• sneaks the RDS file(s) into tempdir() for available.packages to use.
• filtered package list is cached as well

[codecov-io]

Codecov Report

Merging #19 into master will increase coverage by 17.25%. The diff coverage is 45.36%.

@@             Coverage Diff             @@
##           master      #19       +/-   ##
===========================================
+ Coverage       0%   17.25%   +17.25%     
===========================================
  Files          12       14        +2     
  Lines         317      510      +193     
===========================================
+ Hits            0       88       +88     
- Misses        317      422      +105

Impacted Files	Coverage Δ
R/metadata-cache.R	0% <0%> (ø)
R/available-packages.R	0% <0%> (ø)	:arrow_up:
R/utils.R	4.16% <14.28%> (+4.16%)	:arrow_up:
R/downloads.R	87% <87%> (ø)

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