Strip trailing slashes in JULIA_DEPOT_PATH when embedding @depot

[staticfloat]

The new relocatable cache file work uses simple text substitution when stripping out the depot from a cache file's paths, and when substituting it in again over the @depot marker. However, if a user starts julia with JULIA_DEPOT_PATH=/opt/foo/, the embedded path for Foo.jl's includes list will look like @depotpackages/Foo/XYZ/src/Foo.jl, and if the user then uses JULIA_DEPOT_PATH=/opt/foo (which should be equivalent) the cache file will fail to load with the message:

Failed to determine depot from srctext files

This commit standardizes the serialization format to always contain a trailing pathsep(), so that textual substitution is more likely to work regardless of slightly-inconsistent JULIA_DEPOT_PATH settings.

[fatteneder]

Thanks for spotting. Changes look good!

I checked the replace logic again and I think there are at least two more edges cases where we could mess up (but for another reason):

1. When inserting @depot:

   julia/base/loading.jl

   Line 2604 in 837522a

   path = replace(path, depot => "@depot")

   This can yield double substitution when JULIA_DEPOT_PATH=foo/bar and path = "/foo/bar/foo/bar" which would result in @depot/@depot.

2. When replacing @depot: I wanted to be smart and inserted a ^ into the regex to limit substitution to first occurence

   julia/base/loading.jl

   Line 2622 in 837522a

   isfile(replace(inc, r"^@depot" => depot))

   However, this can also give a double substitution in case we have serialized a path like @depot/foo/bar/\n@depot/mypkg/src/file.jl (the second @depot must not necessarily come from case 1. above) and the \n trips up the ^ in the regex.

Both cases should be fixable by adding a count=1 keyword to the relevant replace calls.
You could add this here too, or shall we open another PR?

[staticfloat]

Great comments both of you; I've integrated the suggestions here. I didn't create any test cases for those other edge cases, @fatteneder if you have some spare cycles, would you mind working up a test case for those?