Make internal/apmgodog its own module

[axw]

Move feature test-related code to its own module, so we can add dependencies on instrumentation modules. This is tidier anyway, since it means godog can be removed from the main go.mod.

This is required for some changes needed to get #898 passing.

[codecov-io]

Codecov Report

Merging #899 (19a900e) into master (ba58cd6) will increase coverage by 0.19%.
The diff coverage is 91.48%.

@@            Coverage Diff             @@
##           master     #899      +/-   ##
==========================================
+ Coverage   83.70%   83.89%   +0.19%     
==========================================
  Files         142      140       -2     
  Lines        7934     7868      -66     
==========================================
- Hits         6641     6601      -40     
+ Misses        885      871      -14     
+ Partials      408      396      -12     

Impacted Files	Coverage Δ
modelwriter.go	96.62% <ø> (-0.12%)	⬇️
tracer.go	89.04% <50.00%> (-0.02%)	⬇️
config.go	72.29% <100.00%> (+1.12%)	⬆️
context.go	94.84% <100.00%> (+0.28%)	⬆️
error.go	95.18% <100.00%> (+0.01%)	⬆️
module/apmecho/middleware.go	88.76% <100.00%> (+0.12%)	⬆️
module/apmgin/middleware.go	86.76% <100.00%> (+0.19%)	⬆️
module/apmgorilla/middleware.go	83.78% <100.00%> (+0.45%)	⬆️
module/apmhttp/handler.go	78.90% <100.00%> (+0.16%)	⬆️
module/apmhttp/ignorer.go	55.55% <100.00%> (+5.55%)	⬆️
... and 8 more

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[apmmachine]

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Build stats

• Build Cause: Branch indexing

• Start Time: 2021-02-16T17:20:23.606+0000

• Duration: 28 min 39 sec

• Commit: 19a900e

Test stats 🧪

Test	Results
Failed	0
Passed	7670
Skipped	188
Total	7858

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