Simplify the behavior of package db flags.
Previously, the `-no-user-package` and `-no-global-package` flags affected the "initial" stack only, while `user-package` and `global-packages` appended to the end of the stack. This commit changes the behavior of those flags, so that they are always applied to the stack as a whole. The effect of the GHC_PACKAGE_PATH environment variable has also been changed: terminating it with a separator now adds the default package dbs (user and global) instead of the initial stack.
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