Fix #1032: Correctly test full scalajs lib size again.

The scalajs library used to be in the `externalDependencyClasspath`
even for internal projects, due to the `useLibraryButDoNotDependOnIt`
hack. This hack was removed in commit
2e90a7b259de38ec3a19b27a37abfc759e99215a and the scalajs library moved
to the `internalDependencyClasspath`.

This commit updates the size checking scripts to test both the extdeps
and the intdeps.
Merge pull request #1041 from gzm0/fix-extdeps-check

Fix #1032: Correctly test full scalajs lib size again.