Dynamic library/include paths (#8136)

Fixes #7855
Closes #8070
Closes #2645

When searching for library directories (e.g. to add "-L" arguments to
the compiler wrapper) Spack was only trying the "lib/" and "lib64/"
directories for each dependency install prefix; this missed cases
where packages would install libraries to subdirectories and also was
not customizable. This PR makes use of the ".headers" and ".libs"
properties for more-advanced location of header/library directories.
Since packages can override the default behavior of ".headers" and
".libs", it also allows package writers to customize.

The following environment variables which used to be set by Spack
for a package build have been removed:

* Remove SPACK_PREFIX and SPACK_DEPENDENCIES environment variables as
  they are no-longer used
* Remove SPACK_INSTALL environment variable: it was not used before
  this PR

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