Eliminate some N^2 performance in type checking

Where N is the number of members of the enclosing package.

Double definition errors for top level classes/objects are issued
elsewhere, as demonstrated by the enclosed test. So we can
omit the call to `checkNoDoubleDefs` in this content.

We can't omit the call to `addSynthetics` for package
class owners (case- and value-class synthetic companions
are added here), but we can make the process cheaper by
moving the expensive-but-usually-true call to `shouldAdd`.

Here's an example of the improvement.

    % rm -rf /tmp/pkg; (for i in {1..50}; do for j in {1..100}; do echo "package pkg { class A_${i}_${j}___(val a: Int, val b: Int) }"; done; done) > sandbox/A1.scala && time scalac-hash v2.11.0 -Ybackend:GenASM -J-Xmx1G -J-XX:MaxPermSize=400M -d /tmp sandbox/A1.scala;

    real    0m49.762s
    user    1m12.376s
    sys 0m2.371s

    % rm -rf /tmp/pkg; (for i in {1..50}; do for j in {1..100}; do echo "package pkg { class A_${i}_${j}___(val a: Int, val b: Int) }"; done; done) > sandbox/A1.scala && time qbin/scalac -Ybackend:GenASM -J-Xmx1G -J-XX:MaxPermSize=400M -d /tmp sandbox/A1.scala;

    real    0m35.662s
    user    0m58.275s
    sys 0m2.355s

We've still got another source of pathological performance in
creating nested scopes that I'll fix in the next commit.

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