refactor: merge opentelemetry-base to opentelemetry-core

[legendecas]

Which problem is this PR solving?

• Fixes merge base into api #968
• They act as a very similar role in the SDK.
• ExportResult is a type used in Exporter API yet has to be imported from an additional package to @opentelemetry/api and @opentelemetry/core.

Short description of the changes

• merged @opentelemetry/base to @opentelemetry/core

[mayurkale22]

Please fix the build.

[codecov-io]

Codecov Report

Merging #991 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #991      +/-   ##
==========================================
+ Coverage   94.90%   94.92%   +0.02%     
==========================================
  Files         212      212              
  Lines        8676     8671       -5     
  Branches      791      792       +1     
==========================================
- Hits         8234     8231       -3     
+ Misses        442      440       -2     

Impacted Files	Coverage Δ
packages/opentelemetry-core/src/ExportResult.ts	100.00% <ø> (ø)
...s/opentelemetry-core/src/platform/node/sdk-info.ts	100.00% <ø> (ø)
...s/opentelemetry-metrics/src/export/NoopExporter.ts	33.33% <ø> (ø)
packages/opentelemetry-metrics/src/export/types.ts	100.00% <ø> (ø)
...ages/opentelemetry-core/src/platform/node/index.ts	100.00% <100.00%> (ø)
...emetry-exporter-collector/src/CollectorExporter.ts	90.24% <100.00%> (-0.24%)	⬇️
.../opentelemetry-exporter-collector/src/transform.ts	95.71% <100.00%> (-0.07%)	⬇️
...ckages/opentelemetry-exporter-jaeger/src/jaeger.ts	89.79% <100.00%> (-0.21%)	⬇️
.../opentelemetry-exporter-jaeger/test/jaeger.test.ts	100.00% <100.00%> (ø)
...pentelemetry-exporter-prometheus/src/prometheus.ts	92.00% <100.00%> (-0.08%)	⬇️
... and 13 more

[legendecas]

@mayurkale22 Test fixed :)

[vmarchaud]

lgtm