Remove ArgumentError() in parse_cache_header() when @depot cannot be resolved

In the original implementation of relocatable package cache files,
`parse_cache_header()` was made responsible for resolving
`@depot/`-relative paths, and it threw an `ArgumentError()` if it could
not find one.  However, this could happen for a few reasons:

1. The `JULIA_DEPOT_PATH` could be set incorrectly, so the appropriate
   source text files could not be found in an appropriate `packages/`
   directory.

2. The package could have been upgraded, leaving a cache file for an
   older version on-disk, but no matching source files.

For a concrete example of (2), see the following `bash` script:

    #!/bin/bash
    set -euo pipefail

    TEMP_PATH=$(mktemp -d)
    JULIA=${JULIA:-julia}
    echo JULIA: ${JULIA}
    export JULIA_DEPOT_PATH="${TEMP_PATH}"
    trap 'rm -rf ${TEMP_PATH}' EXIT

    # Create a `Foo.jl` that has an `internal.jl` within it
    FOO_DIR=${JULIA_DEPOT_PATH}/dev/Foo
    mkdir -p ${FOO_DIR}/src
    cat >${FOO_DIR}/Project.toml <<EOF
    name = "Foo"
    uuid = "00000000-0000-0000-0000-000000000001"
    version = "1.0.0"
    EOF
    cat >${FOO_DIR}/src/Foo.jl <<EOF
    module Foo

    include("internal.jl")
    end
    EOF

    cat >${FOO_DIR}/src/internal.jl <<EOF
    # Nothing to see here, folks
    EOF

    ${JULIA} -e "import Pkg; Pkg.develop(\"Foo\"); Pkg.precompile()"

    # Change `Foo` to no longer depend on `internal.jl`; this should invalidate its cache files
    cat >${FOO_DIR}/src/Foo.jl <<EOF
    module Foo
    end
    EOF
    rm -f ${FOO_DIR}/src/internal.jl

    # This should print `SUCCESS!`, not `FAILURE!`
    ${JULIA} -e 'using Foo; println("SUCCESS!")' || echo "FAILURE!"

This pull request removes the `ArgumentError()`, as it seems reasonable
to require `parse_cache_header()` to not throw errors in cases like
these.  We instead rely upon a higher level validation to reject a
cachefile whose depot cannot be found, which should happen when
`stale_cachefile()` later checks to ensure that all includes are found
on-disk, (which will be false, as these paths start with `@depot/`, an
unlikely path prefix to exist).

Author: staticfloat

base/loading.jl

@@ -2718,11 +2718,9 @@ function parse_cache_header(f::IO, cachefile::AbstractString)
         chi.filename ∈ srcfiles && push!(keepidx, i)
     end
     if depot === :no_depot_found
-        throw(ArgumentError("""
-              Failed to determine depot from srctext files in cache file $cachefile.
-              - Make sure you have adjusted DEPOT_PATH in case you relocated depots."""))
+        @debug("Unable to resolve @depot tag in cache file $cachefile", srcfiles)
     elseif depot === :missing_depot_tag
-        @debug "Missing @depot tag for include dependencies in cache file $cachefile."
+        @debug("Missing @depot tag for include dependencies in cache file $cachefile.", srcfiles)
     else
         for inc in includes
             inc.filename = replace(inc.filename, r"^@depot" => depot)