- remove the old LGPL license headers from all files in Spack
- add SPDX headers to all files
- core and most packages are (Apache-2.0 OR MIT)
- a very small number of remaining packages are LGPL-2.1-only
* edits to address issues where spack concretization attempts to set properties on already-installed specs
* most added checks only need to check if the spec is concrete; they dont also need to check if the package is installed
* add test to ensure that patches are not applied to an installed spec
* add test to ensure that an error is detected when a dependent requests a dependency constraint which conflicts with a requested installed dependency
