Better rule loading

[owen-d]

Fixes an issue where the underlying prometheus rulefmt loader parsed expressions as promql -- now we rip out the bits we need for a logql version.

[codecov-commenter]

Codecov Report

Merging #2576 into master will increase coverage by 0.08%.
The diff coverage is 80.43%.

@@            Coverage Diff             @@
##           master    #2576      +/-   ##
==========================================
+ Coverage   62.89%   62.97%   +0.08%     
==========================================
  Files         169      169              
  Lines       14911    15003      +92     
==========================================
+ Hits         9378     9448      +70     
- Misses       4788     4798      +10     
- Partials      745      757      +12     

Impacted Files	Coverage Δ
pkg/ruler/manager/compat.go	47.13% <80.43%> (+47.13%)	⬆️
pkg/promtail/targets/file/tailer.go	70.83% <0.00%> (-4.17%)	⬇️
pkg/promtail/targets/file/filetarget.go	66.85% <0.00%> (-2.29%)	⬇️
pkg/logql/evaluator.go	92.88% <0.00%> (+0.40%)	⬆️
pkg/querier/queryrange/downstreamer.go	97.93% <0.00%> (+2.06%)	⬆️

[slim-bean]

LGTM