[equinox-dev] [p2] repo refactoring

[Jeff McAffer <Jeff_McAffer@xxxxxxxxxx>]

I just released some more repo refactorings.
 some hightlights
- API clarification and simplification.
 After much discussion with John and Pascal we decided to merge back
the writable repos sturcture.  It was just getting to be too much.
 Now rather than adapting you can use isModifiable().  We also
merged in the IRepositoryInfo interfaces.  again for client-side simplification.

- Introduced some explicit SPI.  To
be clear, SPI are effectively API that are aimed at people plugging in
different implementations.  For example, people providing artifact
repo implementations have to implement various interfaces etc.  At
least some of those interfaces are SPI.  SPI-ness does NOT inherently
mean that we can change them willy-nilly after release.  While generally
there are fewer consumers of SPI, we should not play fast and loose with
them.

- Note that the SPI package naming convention
follows the standard pattern of org.eclipse.equinox.spi.p2....  that
is, it is the same as the "internal" pattern but with "spi"
rather than "internal"

Jeff