Currently `spack env activate --with-view` exists, but is a no-op.
So, it is not too much of a breaking change to make this redundant flag
accept a value `spack env activate --with-view <name>` which activates
a particular view by name.
The view name is stored in `SPACK_ENV_VIEW`.
This also fixes an issue where deactivating a view that was activated
with `--without-view` possibly removes entries from PATH, since now we
keep track of whether the default view was "enabled" or not.
bd165ebc4d