Organize API into Go modules based on stability and dependencies

[punya]

Fixes #1467.

This PR

1. Create modules at otel/metric, otel/trace, otel/oteltest, otel/sdk/export/metric, otel/sdk/metric (lots of cyclic dependencies).
2. Move GetMeterProvider, Meter and SetMeterProvider from otel to otel/metric/global.

Future work

1. Mark otel, otel/trace, otel/sdk as v1.0.
2. In future when we release Metrics as a stable signal, leave the meter-related functions in otel/metric - at most, add funcs in otel that forward to them.

---

Reviewers: I recommend looking at this commit-by-commit instead of as a combined diff.

[codecov]

Codecov Report

Merging #1528 (3da34ae) into main (e50a1c8) will decrease coverage by 1.2%.
The diff coverage is 0.0%.

@@           Coverage Diff           @@
##            main   #1528     +/-   ##
=======================================
- Coverage   79.0%   77.7%   -1.3%     
=======================================
  Files        127     127             
  Lines       6683    6683             
=======================================
- Hits        5280    5195     -85     
- Misses      1148    1244     +96     
+ Partials     255     244     -11     

Impacted Files	Coverage Δ
exporters/metric/prometheus/prometheus.go	64.8% <0.0%> (ø)
exporters/stdout/exporter.go	17.2% <0.0%> (ø)
metric/global/metric.go	100.0% <ø> (ø)
internal/matchers/expecter.go	0.0% <0.0%> (-100.0%)	⬇️
internal/internaltest/errors.go	0.0% <0.0%> (-100.0%)	⬇️
sdk/export/metric/aggregation/aggregation.go	0.0% <0.0%> (-100.0%)	⬇️
sdk/export/metric/metric.go	36.8% <0.0%> (-60.6%)	⬇️
sdk/export/metric/exportkind_string.go	0.0% <0.0%> (-57.2%)	⬇️
trace/config.go	75.8% <0.0%> (-24.2%)	⬇️
internal/matchers/expectation.go	0.0% <0.0%> (-19.0%)	⬇️
... and 4 more