Allow `CompositePropertySource` to be constructed from existing property sources

[sbrannen]

Overview

CompositePropertySource currently supports adding individual PropertySource instances via its addPropertySource(PropertySource<?>) and addFirstPropertySource(PropertySource<?>) methods; however, there is no way to construct a new CompositePropertySource from an existing collection of property sources, such as an instance of MutablePropertySources.

To support such use cases, we should introduce a CompositePropertySource(String, Iterable<PropertySource<?>>) constructor, where Iterable<PropertySource<?>> supports collections as well as PropertySources implementations.

Related Issues

• PropertySourcesPlaceholderConfigurer placeholder resolution fails in several scenarios #34861

[sbrannen]

This change has been reverted in 65e3013, since we no longer think it will be necessary.