I noticed that `abseil-cpp` was showing in `spack find` with "no compiler", and the only
difference between it and other nodes was that it *only* depends on `cxx` -- others
depend on `c` as well.
It turns out that the `select()` method on `EdgeMap` only takes `Sequence[str]` and doesn't
check whether they're actually just one `str`. So asking for, e.g., `cxx` is like asking for
`c` or `x` or `x`, as the `str` is treated like a sequence. This causes Spack to miss `cxx`
and `fortran` language virtuals in `DeprecatedCompilerSpec`.
Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>
3444d40ae2