native.bbclass: avoid unintended substring replacement when setting PROVIDES
The way native_virtclass_handler was implemented leaded to
unintended substring replacements when setting PROVIDES for
native providers, in case the original PROVIDES value contains
providees with common substrings.
Here's a practical case where the old behavior was problematic:
the oracle-jse-jdk-x86-64 recipe provides both virtual/java and
virtual/javac:
