[MRESOLVER-572] Export internal packages in OSGi metadata as x-internal

This makes maven-resolver-supplier usable as bundle in an OSGi runtime.
At the moment the bundle cannot resolve because the internal.impl
packages it uses and imports in its OSGi metadata are not exported by
maven-resolver-impl.

Simply exporting internal packages fixes that and, although it is
discouraged to use internal/impl packages, gives consumers that can
accept no API-compatibility guarantees the ability to access these
packages.
Adding the directive 'x-internal' is a convention that, at least when
using Eclipse PDE, shows a corresponding warning at call-sides.

Fixes https://issues.apache.org/jira/browse/MRESOLVER-572

pom.xml

@@ -475,7 +475,10 @@
               Bundle-SymbolicName: org.apache.${replacestring;${project.artifactId};-;.}
               Automatic-Module-Name: ${Bundle-SymbolicName}
               # Export packages not containing the substring 'internal'
-              -exportcontents: ${removeall;${packages};${packages;NAMED;*internal*}}
+              -exportcontents: \
+                *.impl.*;x-internal:=true, \
+                *.internal.*;x-internal:=true, \
+                *
               # Reproducible build
               -noextraheaders: true
               ${bnd.instructions.additions}