If any of the sources has a @PropertySource annotation (or many)
then we can add those properties to the Environment. It's a nice
convenient way of specifying a custom external properties location
for an app.

One problem is that Spring will come along and parse the same
annotations later as part of the @Configuration parsing. The
user has pretty limited control over how that is done, and it
will never be done in a "natural" way for a Boot application
(which would prefer that the default application.properties
is applied *last*, whereas Spring will apply the @PropertySource
last).

To get round that problem we add the property sources with
a different name (key in the PropertySources in Environment),
prefixing named property sources with "boot.", and adding
others with a name that is the same as the resource location
(instead of its description, which is the default for
Spring).

Another problem is that Spring doesn't know about YAML, so
the user is currently restricted to using properties files
with this annotation.