base.pm no longer modifies $VERSION

base.pm no longer modifies $VERSION

Previously, base.pm modified the $VERSION of modules it loaded to
the string "-1, set by base.pm".  This is not a valid lax version
string and thus could not be parsed by version.pm. (It is also an
encapsulation violation, as it modifies a global in another package.)

This patch removes the $VERSION modification code entirely and uses
a private hash to track which modules base.pm has successfully loaded.
This also eliminates a subtle bug in how base.pm was checking for
the existence of a package's VERSION scalar.

[Though the final mechanism is different, thank you to John Peacock for
proposing the initial patch to eliminate the "-1..." code from base.pm]