Change the installer to take `([pkg], args)` in the constructor instead
of `[(pkg, args)]`. The reason is that certain arguments are global
settings, and the new API ensures that those arguments cannot be
different across different "build requests".
The `explicit` install arg is now a list of hashes, and the installer is
no longer responsible for determining what package is installed
explicitly. This way environment installs can simply pass the list of
environment roots, without them necessarily being explicit build
requests. For example an env with two roots [a, b], where b depends on
a, would not always cause spack install to mark b as explicit.
Notice that `overwrite` already took a list of hashes, this makes
`explicit` consistent.
`package.do_install(explicit=True)` continues to take a boolean.
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