Dynamic migration dicovery, bug fix #460

Job	Run time
Build docs	1m 15s
cargo-audit	11s
Cargo fmt and clippy	1m 31s
set-rust-versions	2s
Test postgres (stable)	1m 40s
Test postgres (nightly)	1m 54s
Test postgres (1.75)	1m 46s
Test mysql-async (stable)	1m 3s
test-macros-and-cli (stable, windows-latest)	3m 27s
Test mysql-async (nightly)	1m 11s
Test mysql (stable)	2m 10s
test-macros-and-cli (stable, ubuntu-latest)	2m 1s
Test mysql-async (1.75)	1m 16s
Test Sqlite (stable)	1m 33s
test-macros-and-cli (nightly, windows-latest)	4m 1s
Test mysql (nightly)	2m 13s
Test Sqlite (nightly)	1m 40s
test-macros-and-cli (nightly, ubuntu-latest)	2m 4s
Test mysql (1.75)	2m 21s
Test Sqlite (1.75)	1m 41s
test-macros-and-cli (1.75, windows-latest)	4m 8s
Test tiberius (stable)	1m 48s
test-macros-and-cli (1.75, ubuntu-latest)	2m 10s
Test tiberius (nightly)	2m 5s
Test tokio-postgres (stable)	46s
Test tiberius (1.75)	2m 3s
Test tokio-postgres (nightly)	1m 4s
Test tokio-postgres (1.75)	1m 4s
CI is green	1s
	50m 9s

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