split Directory.Build file into two dedicated files, one for src (packages) and one for test projects

[adamsitnik]

#2185 inspired me to do a little bit of MSBuild refactor: split Directory.Build file into two dedicated files, one for src (packages) and one for test projects. This allows us the reduce the number of conditions in the props file

[eerhardt]

I had to force myself not to do this as part of my PR. 😆

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.72%. Comparing base (14256f3) to head (82b2211).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2188      +/-   ##
==========================================
- Coverage   66.07%   64.72%   -1.36%     
==========================================
  Files         153      258     +105     
  Lines        3499     8515    +5016     
  Branches      367      604     +237     
==========================================
+ Hits         2312     5511    +3199     
- Misses       1111     2852    +1741     
- Partials       76      152      +76     

Flag	Coverage Δ
ArangoDb	28.30% <ø> (ø)
Aws.S3	15.52% <ø> (ø)
Aws.Sns	15.97% <ø> (ø)
AzureApplicationInsights	16.56% <ø> (ø)
AzureBlobStorage	26.42% <ø> (ø)
AzureDigitalTwin	37.15% <ø> (ø)
AzureEventHubs	17.46% <ø> (ø)
AzureFileStorage	?
AzureQueueStorage	?
AzureSearch	17.60% <ø> (ø)
AzureTableStorage	27.97% <ø> (ø)
Consul	24.32% <ø> (ø)
Dapr	14.50% <ø> (?)
DocumentDb	16.00% <ø> (ø)
DynamoDb	13.29% <ø> (ø)
Elasticsearch	41.12% <ø> (ø)
EventStore.gRPC	25.51% <ø> (?)
Gremlin	25.00% <ø> (ø)
Hangfire	12.10% <ø> (ø)
IbmMQ	30.76% <ø> (ø)
InfluxDB	15.54% <ø> (ø)
Kafka	23.52% <ø> (ø)
Kubernetes	41.54% <ø> (?)
MySql	33.33% <ø> (ø)
Nats	72.77% <ø> (ø)
Npgsql	42.71% <ø> (?)
Oracle	63.92% <ø> (ø)
Publisher.CloudWatch	20.33% <ø> (ø)
Publisher.Prometheus	18.75% <ø> (?)
Publisher.Seq	40.30% <ø> (?)
RabbitMQ	49.56% <ø> (?)
RavenDb	73.42% <ø> (ø)
Redis	68.47% <ø> (ø)
SendGrid	13.19% <ø> (ø)
SignalR	25.97% <ø> (ø)
SqlServer	30.43% <ø> (ø)
Sqlite	28.20% <ø> (ø)
System	43.58% <ø> (ø)
Uris	0.00% <ø> (?)

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