Fix missing spack.compiler import (#8240)

Fixes: #8258

#8090 altered import behavior so that import spack no longer
provides access to many other Spack modules. This addresses
a case which depended on the prior behavior and was not
updated as part of #8090. This particular import error only
came up when users were setting compiler flags on specs.

See also: #8194

lib/spack/spack/spec.py

@@ -115,6 +115,7 @@
 from llnl.util.tty.color import cwrite, colorize, cescape, get_color_when
 
 import spack.architecture
+import spack.compiler
 import spack.compilers as compilers
 import spack.error
 import spack.parse