Commit 3295289e17 fixed a number issues with placeholder resolution in
PropertySourcesPlaceholderConfigurer. However, in doing so, it replaced
a raw PropertySource with a CompositePropertySource which implements
EnumerablePropertySource.
Consequently, all property sources registered in the Environment must
now implement EnumerablePropertySource (which is not an actual
requirement). Otherwise, invocations of getPropertyNames() on the
CompositePropertySource result in an IllegalStateException, and that is
a breaking change which resulted in numerous build failures within the
Spring portfolio.
To address that regression, this commit introduces a private
ConfigurableEnvironmentPropertySource in
PropertySourcesPlaceholderConfigurer which is a "raw" PropertySource
that delegates directly to the PropertySources in a
ConfigurableEnvironment.
This commit also extracts the raw PropertySource for direct Environment
delegation into a new FallbackEnvironmentPropertySource.
See gh-17385
Closes gh-34861
ebb44a8368