Add license directive (#39346)

This patch adds in a license directive to get the ball rolling on adding in license 
information about packages to spack. I'm primarily interested in just adding
license into spack, but this would also help with other efforts that people are
interested in such as adding license information to the ASP solve for 
concretization to make sure licenses are compatible.

Usage:

Specifying the specific license that a package is released under in a project's
`package.py` is good practice. To specify a license, find the SPDX identifier for
a project and then add it using the license directive:

```python
   license("<SPDX Identifier HERE>")
```

For example, for Apache 2.0, you might write:

```python
   license("Apache-2.0")
```

Note that specifying a license without a when clause makes it apply to all
versions and variants of the package, which might not actually be the case.
For example, a project might have switched licenses at some point or have
certain build configurations that include files that are licensed differently.
To account for this, you can specify when licenses should be applied. For
example, to specify that a specific license identifier should only apply
to versionup to and including 1.5, you could write the following directive:

```python
   license("MIT", when="@:1.5")
```

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