command/init: Fix TestInit_providerSource
command/init: Fix TestInit_providerSource

This test is focused on making sure that the required_providers syntax
is working, so the rewritten version does not include any special handling
of pre-installed packages or "vendored" packages. Pre-installed plugins
are tested in other tests such as TestInit_getUpgradePlugins.

rnaveiras/terraform,VladRassokhin/terraform,apparentlymart/terraform,hashicorp/terraform,pdecat/terraform,VladRassokhin/terraform,tommynsong/terraform,apparentlymart/terraform,tommynsong/terraform,apparentlymart/terraform,heimweh/terraform,heimweh/terraform,pdecat/terraform,heimweh/terraform,hashicorp/terraform,ewbankkit/terraform,rnaveiras/terraform,hashicorp/terraform,VladRassokhin/terraform,VladRassokhin/terraform,pryorda/terraform,ewbankkit/terraform,heimweh/terraform,pryorda/terraform,heimweh/terraform,pryorda/terraform,ewbankkit/terraform,rnaveiras/terraform,tommynsong/terraform,apparentlymart/terraform,ewbankkit/terraform,apparentlymart/terraform,pdecat/terraform,pdecat/terraform