fix(broker): use 'pb:pacts' instead 'pacts' (broker v2.19.0)

[gaelollivier]

This fixes a bug introduced in pact broker v2.19.0 (pact-foundation/pact_broker#209) which results in only the latest consumer being verified when running provider verification tests.

[mefellows]

Thanks @gaelduplessix, if you could get those tests to pass i'll bring this straight in.

[gaelollivier]

• Fixed the broker tests, now I get installer_test.go:42: error: exec: "pact-mock-service": executable file not found in %PATH%, not sure how to fix this 🤔
• I realised that this PR will actually make the go-client require pact-broker >v2.19.0, is that ok ? Should probably be written in the released notes

[gaelollivier]

While I was at it, I cherry-picked 2e0fc84 on my PR because my tests were failing now that they're properly loading multiple pacts ^^'

[mefellows]

@gaelduplessix apologies, feat/daemonless is a defunct branch. I'll move all of the active code into release/1.x.x which will be the next major version. These changes are already in it.